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This function has to ensure it doesn't follow a symlink that could be used to escape the virtfs directory. This could be easily achieved if fchmodat() on linux honored the AT_SYMLINK_NOFOLLOW flag as described in POSIX, but it doesn't. There was a tentative to implement a new fchmodat2() syscall with the correct semantics: https://patchwork.kernel.org/patch/9596301/ but it didn't gain much momentum. Also it was suggested to look at an O_PATH based solution in the first place. The current implementation covers most use-cases, but it notably fails if: - the target path has access rights equal to 0000 (openat() returns EPERM), => once you've done chmod(0000) on a file, you can never chmod() again - the target path is UNIX domain socket (openat() returns ENXIO) => bind() of UNIX domain sockets fails if the file is on 9pfs The solution is to use O_PATH: openat() now succeeds in both cases, and we can ensure the path isn't a symlink with fstat(). The associated entry in "/proc/self/fd" can hence be safely passed to the regular chmod() syscall. The previous behavior is kept for older systems that don't have O_PATH. Signed-off-by: Greg Kurz <groug@kaod.org> Reviewed-by: Eric Blake <eblake@redhat.com> Tested-by: Zhi Yong Wu <zhiyong.wu@ucloud.cn> Acked-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
65 lines
1.5 KiB
C
65 lines
1.5 KiB
C
/*
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* 9p utilities
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*
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* Copyright IBM, Corp. 2017
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*
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* Authors:
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* Greg Kurz <groug@kaod.org>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*/
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#ifndef QEMU_9P_UTIL_H
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#define QEMU_9P_UTIL_H
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#ifdef O_PATH
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#define O_PATH_9P_UTIL O_PATH
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#else
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#define O_PATH_9P_UTIL 0
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#endif
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static inline void close_preserve_errno(int fd)
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{
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int serrno = errno;
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close(fd);
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errno = serrno;
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}
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static inline int openat_dir(int dirfd, const char *name)
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{
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return openat(dirfd, name,
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O_DIRECTORY | O_RDONLY | O_NOFOLLOW | O_PATH_9P_UTIL);
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}
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static inline int openat_file(int dirfd, const char *name, int flags,
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mode_t mode)
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{
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int fd, serrno, ret;
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fd = openat(dirfd, name, flags | O_NOFOLLOW | O_NOCTTY | O_NONBLOCK,
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mode);
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if (fd == -1) {
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return -1;
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}
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serrno = errno;
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/* O_NONBLOCK was only needed to open the file. Let's drop it. We don't
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* do that with O_PATH since fcntl(F_SETFL) isn't supported, and openat()
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* ignored it anyway.
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*/
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if (!(flags & O_PATH_9P_UTIL)) {
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ret = fcntl(fd, F_SETFL, flags);
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assert(!ret);
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}
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errno = serrno;
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return fd;
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}
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ssize_t fgetxattrat_nofollow(int dirfd, const char *path, const char *name,
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void *value, size_t size);
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int fsetxattrat_nofollow(int dirfd, const char *path, const char *name,
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void *value, size_t size, int flags);
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#endif
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