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9pfs: local: clarify fchmodat_nofollow() implementation

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Since fchmodat(2) on Linux doesn't support AT_SYMLINK_NOFOLLOW, we have to
implement it using workarounds. There are two different ways, depending on
whether the system supports O_PATH or not.

In the case O_PATH is supported, we rely on the behavhior of openat(2)
when passing O_NOFOLLOW | O_PATH and the file is a symbolic link. Even
if openat_file() already adds O_NOFOLLOW to the flags, this patch makes
it explicit that we need both creation flags to obtain the expected
behavior.

This is only cleanup, no functional change.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
This commit is contained in:
Greg Kurz 2017-09-04 09:24:53 +02:00
parent 3c08f4a433
commit aa5e85a108
1 changed files with 8 additions and 4 deletions
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12
hw/9pfs/9p-local.c
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@ -349,11 +349,11 @@ static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
return -1;
}
/* Access modes are ignored when O_PATH is supported. We try O_RDONLY and
* O_WRONLY for old-systems that don't support O_PATH.
*/
fd = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL, 0);
fd = openat_file(dirfd, name, O_RDONLY | O_PATH_9P_UTIL | O_NOFOLLOW, 0);
#if O_PATH_9P_UTIL == 0
/* Fallback for systems that don't support O_PATH: we depend on the file
* being readable or writable.
*/
if (fd == -1) {
/* In case the file is writable-only and isn't a directory. */
if (errno == EACCES) {
@ -368,6 +368,10 @@ static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode)
}
ret = fchmod(fd, mode);
#else
/* Access modes are ignored when O_PATH is supported. If name is a symbolic
* link, O_PATH | O_NOFOLLOW causes openat(2) to return a file descriptor
* referring to the symbolic link.
*/
if (fd == -1) {
return -1;
}