[PATCH] Fix `statx()` emulation breaking exports

[Patrick Steinhardt <ps@pks.im>]

[-- Attachment #1: Type: text/plain, Size: 1717 bytes --]

Ever since commit 76c21e3f (mountd: Check the stat() return values in
match_fsid(), 2020-05-08), it wasn't possible to export filesystems
on my musl based system anymore.

The root cause of this is the innocuous-looking change to decide based
on `errno` whether `is_mountpoint()` raised a real error or whether it
simply didn't match. The issue is that `is_mountpoint()` transitively
calls into our `xlstat()` wrapper, which either executes `statx()` if
the system supports it or otherwise falls back to `fstatat()`. But if
`statx()` is not supported, then we'll always first set `errno = ENOSYS`
before calling `fstatat()`. So effectively, all systems which do not
have `statx()` and whose `fstatat()` doesn't reset `errno` will cause us
to end up with errno set to `ENOSYS`.

Fix the issue by resetting `errno` before calling `fstatat()` in both
`xlstat()` and `xstat()`.

Fixes: 76c21e3f (mountd: Check the stat() return values in match_fsid(), 2020-05-08)
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 support/misc/xstat.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/support/misc/xstat.c b/support/misc/xstat.c
index a438fbcc..6f751f7f 100644
--- a/support/misc/xstat.c
+++ b/support/misc/xstat.c
@@ -85,6 +85,7 @@ int xlstat(const char *pathname, struct stat *statbuf)
 		return 0;
 	else if (errno != ENOSYS)
 		return -1;
+	errno = 0;
 	return fstatat(AT_FDCWD, pathname, statbuf, AT_NO_AUTOMOUNT |
 			AT_SYMLINK_NOFOLLOW);
 }
@@ -95,6 +96,7 @@ int xstat(const char *pathname, struct stat *statbuf)
 		return 0;
 	else if (errno != ENOSYS)
 		return -1;
+	errno = 0;
 	return fstatat(AT_FDCWD, pathname, statbuf, AT_NO_AUTOMOUNT);
 }
 
-- 
2.31.1


[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]