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Date:   Fri, 16 Apr 2021 11:00:49 +0200
From:   Patrick Steinhardt <ps@pks.im>
To:     linux-nfs@vger.kernel.org
Cc:     Trond Myklebust <trond.myklebust@hammerspace.com>
Subject: [PATCH] Fix `statx()` emulation breaking exports
Message-ID: <a5547a1af1dc90d65100c873204a5b0912ecb9a8.1618563564.git.ps@pks.im>
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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 =3D 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(), 20=
20-05-08)
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 !=3D ENOSYS)
 		return -1;
+	errno =3D 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 !=3D ENOSYS)
 		return -1;
+	errno =3D 0;
 	return fstatat(AT_FDCWD, pathname, statbuf, AT_NO_AUTOMOUNT);
 }
=20
--=20
2.31.1


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