mirror of https://github.com/docker/cli.git
517 lines
12 KiB
C
517 lines
12 KiB
C
/*
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* Copyright (C) 2019 Aleksa Sarai <cyphar@cyphar.com>
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* Copyright (C) 2019 SUSE LLC
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#define _GNU_SOURCE
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#include <unistd.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <stdbool.h>
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#include <string.h>
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#include <limits.h>
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#include <fcntl.h>
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#include <errno.h>
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#include <sys/types.h>
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#include <sys/stat.h>
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#include <sys/statfs.h>
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#include <sys/vfs.h>
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#include <sys/mman.h>
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#include <sys/mount.h>
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#include <sys/sendfile.h>
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#include <sys/syscall.h>
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/* Use our own wrapper for memfd_create. */
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#if !defined(SYS_memfd_create) && defined(__NR_memfd_create)
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# define SYS_memfd_create __NR_memfd_create
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#endif
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/* memfd_create(2) flags -- copied from <linux/memfd.h>. */
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#ifndef MFD_CLOEXEC
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# define MFD_CLOEXEC 0x0001U
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# define MFD_ALLOW_SEALING 0x0002U
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#endif
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int memfd_create(const char *name, unsigned int flags)
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{
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#ifdef SYS_memfd_create
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return syscall(SYS_memfd_create, name, flags);
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#else
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errno = ENOSYS;
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return -1;
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#endif
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}
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/* This comes directly from <linux/fcntl.h>. */
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#ifndef F_LINUX_SPECIFIC_BASE
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# define F_LINUX_SPECIFIC_BASE 1024
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#endif
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#ifndef F_ADD_SEALS
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# define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
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# define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
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#endif
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#ifndef F_SEAL_SEAL
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# define F_SEAL_SEAL 0x0001 /* prevent further seals from being set */
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# define F_SEAL_SHRINK 0x0002 /* prevent file from shrinking */
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# define F_SEAL_GROW 0x0004 /* prevent file from growing */
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# define F_SEAL_WRITE 0x0008 /* prevent writes */
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#endif
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#define CLONED_BINARY_ENV "_LIBCONTAINER_CLONED_BINARY"
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#define RUNC_MEMFD_COMMENT "runc_cloned:/proc/self/exe"
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#define RUNC_MEMFD_SEALS \
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(F_SEAL_SEAL | F_SEAL_SHRINK | F_SEAL_GROW | F_SEAL_WRITE)
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static void *must_realloc(void *ptr, size_t size)
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{
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void *old = ptr;
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do {
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ptr = realloc(old, size);
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} while(!ptr);
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return ptr;
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}
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/*
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* Verify whether we are currently in a self-cloned program (namely, is
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* /proc/self/exe a memfd). F_GET_SEALS will only succeed for memfds (or rather
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* for shmem files), and we want to be sure it's actually sealed.
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*/
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static int is_self_cloned(void)
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{
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int fd, ret, is_cloned = 0;
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struct stat statbuf = {};
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struct statfs fsbuf = {};
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fd = open("/proc/self/exe", O_RDONLY|O_CLOEXEC);
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if (fd < 0)
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return -ENOTRECOVERABLE;
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/*
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* Is the binary a fully-sealed memfd? We don't need CLONED_BINARY_ENV for
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* this, because you cannot write to a sealed memfd no matter what (so
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* sharing it isn't a bad thing -- and an admin could bind-mount a sealed
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* memfd to /usr/bin/runc to allow re-use).
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*/
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ret = fcntl(fd, F_GET_SEALS);
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if (ret >= 0) {
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is_cloned = (ret == RUNC_MEMFD_SEALS);
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goto out;
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}
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/*
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* All other forms require CLONED_BINARY_ENV, since they are potentially
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* writeable (or we can't tell if they're fully safe) and thus we must
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* check the environment as an extra layer of defence.
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*/
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if (!getenv(CLONED_BINARY_ENV)) {
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is_cloned = false;
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goto out;
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}
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/*
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* Is the binary on a read-only filesystem? We can't detect bind-mounts in
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* particular (in-kernel they are identical to regular mounts) but we can
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* at least be sure that it's read-only. In addition, to make sure that
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* it's *our* bind-mount we check CLONED_BINARY_ENV.
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*/
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if (fstatfs(fd, &fsbuf) >= 0)
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is_cloned |= (fsbuf.f_flags & MS_RDONLY);
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/*
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* Okay, we're a tmpfile -- or we're currently running on RHEL <=7.6
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* which appears to have a borked backport of F_GET_SEALS. Either way,
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* having a file which has no hardlinks indicates that we aren't using
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* a host-side "runc" binary and this is something that a container
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* cannot fake (because unlinking requires being able to resolve the
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* path that you want to unlink).
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*/
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if (fstat(fd, &statbuf) >= 0)
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is_cloned |= (statbuf.st_nlink == 0);
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out:
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close(fd);
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return is_cloned;
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}
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/* Read a given file into a new buffer, and providing the length. */
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static char *read_file(char *path, size_t *length)
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{
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int fd;
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char buf[4096], *copy = NULL;
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if (!length)
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return NULL;
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fd = open(path, O_RDONLY | O_CLOEXEC);
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if (fd < 0)
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return NULL;
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*length = 0;
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for (;;) {
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ssize_t n;
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n = read(fd, buf, sizeof(buf));
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if (n < 0)
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goto error;
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if (!n)
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break;
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copy = must_realloc(copy, (*length + n) * sizeof(*copy));
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memcpy(copy + *length, buf, n);
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*length += n;
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}
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close(fd);
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return copy;
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error:
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close(fd);
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free(copy);
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return NULL;
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}
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/*
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* A poor-man's version of "xargs -0". Basically parses a given block of
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* NUL-delimited data, within the given length and adds a pointer to each entry
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* to the array of pointers.
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*/
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static int parse_xargs(char *data, int data_length, char ***output)
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{
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int num = 0;
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char *cur = data;
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if (!data || *output != NULL)
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return -1;
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while (cur < data + data_length) {
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num++;
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*output = must_realloc(*output, (num + 1) * sizeof(**output));
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(*output)[num - 1] = cur;
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cur += strlen(cur) + 1;
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}
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(*output)[num] = NULL;
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return num;
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}
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/*
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* "Parse" out argv from /proc/self/cmdline.
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* This is necessary because we are running in a context where we don't have a
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* main() that we can just get the arguments from.
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*/
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static int fetchve(char ***argv)
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{
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char *cmdline = NULL;
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size_t cmdline_size;
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cmdline = read_file("/proc/self/cmdline", &cmdline_size);
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if (!cmdline)
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goto error;
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if (parse_xargs(cmdline, cmdline_size, argv) <= 0)
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goto error;
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return 0;
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error:
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free(cmdline);
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return -EINVAL;
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}
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enum {
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EFD_NONE = 0,
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EFD_MEMFD,
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EFD_FILE,
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};
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/*
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* This comes from <linux/fcntl.h>. We can't hard-code __O_TMPFILE because it
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* changes depending on the architecture. If we don't have O_TMPFILE we always
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* have the mkostemp(3) fallback.
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*/
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#ifndef O_TMPFILE
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# if defined(__O_TMPFILE) && defined(O_DIRECTORY)
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# define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
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# endif
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#endif
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static int make_execfd(int *fdtype)
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{
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int fd = -1;
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char template[PATH_MAX] = {0};
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char *prefix = secure_getenv("_LIBCONTAINER_STATEDIR");
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if (!prefix || *prefix != '/')
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prefix = "/tmp";
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if (snprintf(template, sizeof(template), "%s/runc.XXXXXX", prefix) < 0)
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return -1;
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/*
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* Now try memfd, it's much nicer than actually creating a file in STATEDIR
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* since it's easily detected thanks to sealing and also doesn't require
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* assumptions about STATEDIR.
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*/
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*fdtype = EFD_MEMFD;
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fd = memfd_create(RUNC_MEMFD_COMMENT, MFD_CLOEXEC | MFD_ALLOW_SEALING);
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if (fd >= 0)
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return fd;
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if (errno != ENOSYS && errno != EINVAL)
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goto error;
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#ifdef O_TMPFILE
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/*
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* Try O_TMPFILE to avoid races where someone might snatch our file. Note
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* that O_EXCL isn't actually a security measure here (since you can just
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* fd re-open it and clear O_EXCL).
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*/
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*fdtype = EFD_FILE;
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fd = open(prefix, O_TMPFILE | O_EXCL | O_RDWR | O_CLOEXEC, 0700);
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if (fd >= 0) {
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struct stat statbuf = {};
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bool working_otmpfile = false;
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/*
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* open(2) ignores unknown O_* flags -- yeah, I was surprised when I
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* found this out too. As a result we can't check for EINVAL. However,
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* if we get nlink != 0 (or EISDIR) then we know that this kernel
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* doesn't support O_TMPFILE.
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*/
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if (fstat(fd, &statbuf) >= 0)
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working_otmpfile = (statbuf.st_nlink == 0);
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if (working_otmpfile)
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return fd;
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/* Pretend that we got EISDIR since O_TMPFILE failed. */
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close(fd);
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errno = EISDIR;
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}
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if (errno != EISDIR)
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goto error;
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#endif /* defined(O_TMPFILE) */
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/*
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* Our final option is to create a temporary file the old-school way, and
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* then unlink it so that nothing else sees it by accident.
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*/
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*fdtype = EFD_FILE;
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fd = mkostemp(template, O_CLOEXEC);
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if (fd >= 0) {
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if (unlink(template) >= 0)
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return fd;
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close(fd);
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}
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error:
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*fdtype = EFD_NONE;
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return -1;
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}
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static int seal_execfd(int *fd, int fdtype)
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{
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switch (fdtype) {
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case EFD_MEMFD:
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return fcntl(*fd, F_ADD_SEALS, RUNC_MEMFD_SEALS);
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case EFD_FILE: {
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/* Need to re-open our pseudo-memfd as an O_PATH to avoid execve(2) giving -ETXTBSY. */
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int newfd;
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char fdpath[PATH_MAX] = {0};
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if (fchmod(*fd, 0100) < 0)
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return -1;
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if (snprintf(fdpath, sizeof(fdpath), "/proc/self/fd/%d", *fd) < 0)
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return -1;
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newfd = open(fdpath, O_PATH | O_CLOEXEC);
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if (newfd < 0)
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return -1;
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close(*fd);
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*fd = newfd;
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return 0;
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}
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default:
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break;
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}
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return -1;
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}
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static int try_bindfd(void)
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{
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int fd, ret = -1;
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char template[PATH_MAX] = {0};
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char *prefix = secure_getenv("_LIBCONTAINER_STATEDIR");
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if (!prefix || *prefix != '/')
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prefix = "/tmp";
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if (snprintf(template, sizeof(template), "%s/runc.XXXXXX", prefix) < 0)
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return ret;
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/*
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* We need somewhere to mount it, mounting anything over /proc/self is a
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* BAD idea on the host -- even if we do it temporarily.
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*/
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fd = mkstemp(template);
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if (fd < 0)
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return ret;
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close(fd);
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/*
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* For obvious reasons this won't work in rootless mode because we haven't
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* created a userns+mntns -- but getting that to work will be a bit
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* complicated and it's only worth doing if someone actually needs it.
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*/
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ret = -EPERM;
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if (mount("/proc/self/exe", template, "", MS_BIND, "") < 0)
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goto out;
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if (mount("", template, "", MS_REMOUNT | MS_BIND | MS_RDONLY, "") < 0)
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goto out_umount;
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/* Get read-only handle that we're sure can't be made read-write. */
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ret = open(template, O_PATH | O_CLOEXEC);
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out_umount:
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/*
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* Make sure the MNT_DETACH works, otherwise we could get remounted
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* read-write and that would be quite bad (the fd would be made read-write
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* too, invalidating the protection).
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*/
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if (umount2(template, MNT_DETACH) < 0) {
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if (ret >= 0)
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close(ret);
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ret = -ENOTRECOVERABLE;
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}
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out:
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/*
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* We don't care about unlink errors, the worst that happens is that
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* there's an empty file left around in STATEDIR.
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*/
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unlink(template);
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return ret;
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}
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static ssize_t fd_to_fd(int outfd, int infd)
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{
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ssize_t total = 0;
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char buffer[4096];
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for (;;) {
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ssize_t nread, nwritten = 0;
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nread = read(infd, buffer, sizeof(buffer));
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if (nread < 0)
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return -1;
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if (!nread)
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break;
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do {
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ssize_t n = write(outfd, buffer + nwritten, nread - nwritten);
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if (n < 0)
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return -1;
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nwritten += n;
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} while(nwritten < nread);
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total += nwritten;
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}
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return total;
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}
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static int clone_binary(void)
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{
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int binfd, execfd;
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struct stat statbuf = {};
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size_t sent = 0;
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int fdtype = EFD_NONE;
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/*
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* Before we resort to copying, let's try creating an ro-binfd in one shot
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* by getting a handle for a read-only bind-mount of the execfd.
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*/
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execfd = try_bindfd();
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if (execfd >= 0)
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return execfd;
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/*
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* Dammit, that didn't work -- time to copy the binary to a safe place we
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* can seal the contents.
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*/
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execfd = make_execfd(&fdtype);
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if (execfd < 0 || fdtype == EFD_NONE)
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return -ENOTRECOVERABLE;
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binfd = open("/proc/self/exe", O_RDONLY | O_CLOEXEC);
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if (binfd < 0)
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goto error;
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if (fstat(binfd, &statbuf) < 0)
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goto error_binfd;
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while (sent < statbuf.st_size) {
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int n = sendfile(execfd, binfd, NULL, statbuf.st_size - sent);
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if (n < 0) {
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/* sendfile can fail so we fallback to a dumb user-space copy. */
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n = fd_to_fd(execfd, binfd);
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if (n < 0)
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goto error_binfd;
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}
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sent += n;
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}
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close(binfd);
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if (sent != statbuf.st_size)
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goto error;
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if (seal_execfd(&execfd, fdtype) < 0)
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goto error;
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return execfd;
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error_binfd:
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close(binfd);
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error:
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close(execfd);
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return -EIO;
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}
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/* Get cheap access to the environment. */
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extern char **environ;
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int ensure_cloned_binary(void)
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{
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int execfd;
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char **argv = NULL;
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/* Check that we're not self-cloned, and if we are then bail. */
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int cloned = is_self_cloned();
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if (cloned > 0 || cloned == -ENOTRECOVERABLE)
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return cloned;
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if (fetchve(&argv) < 0)
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return -EINVAL;
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execfd = clone_binary();
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if (execfd < 0)
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return -EIO;
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if (putenv(CLONED_BINARY_ENV "=1"))
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goto error;
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fexecve(execfd, argv, environ);
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error:
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close(execfd);
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return -ENOEXEC;
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}
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