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authorOmar Sandoval <osandov@fb.com>2020-05-14 08:06:43 +0200
committerGreg Kurz <groug@kaod.org>2020-05-14 08:06:43 +0200
commita5804fcf7b22fc7d1f9ec794dd284c7d504bd16b (patch)
tree43f2432d30aa3ed5ef88c1b5663765a6aa0c8269
parent65abaa01ee5781f525e1b9a0d6e6e5a3d8696d5f (diff)
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9pfs: local: ignore O_NOATIME if we don't have permissions
QEMU's local 9pfs server passes through O_NOATIME from the client. If the QEMU process doesn't have permissions to use O_NOATIME (namely, it does not own the file nor have the CAP_FOWNER capability), the open will fail. This causes issues when from the client's point of view, it believes it has permissions to use O_NOATIME (e.g., a process running as root in the virtual machine). Additionally, overlayfs on Linux opens files on the lower layer using O_NOATIME, so in this case a 9pfs mount can't be used as a lower layer for overlayfs (cf. https://github.com/osandov/drgn/blob/dabfe1971951701da13863dbe6d8a1d172ad9650/vmtest/onoatimehack.c and https://github.com/NixOS/nixpkgs/issues/54509). Luckily, O_NOATIME is effectively a hint, and is often ignored by, e.g., network filesystems. open(2) notes that O_NOATIME "may not be effective on all filesystems. One example is NFS, where the server maintains the access time." This means that we can honor it when possible but fall back to ignoring it. Acked-by: Christian Schoenebeck <qemu_oss@crudebyte.com> Signed-off-by: Omar Sandoval <osandov@fb.com> Message-Id: <e9bee604e8df528584693a4ec474ded6295ce8ad.1587149256.git.osandov@fb.com> Signed-off-by: Greg Kurz <groug@kaod.org>
-rw-r--r--hw/9pfs/9p-util.h13
1 files changed, 13 insertions, 0 deletions
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 79ed6b2..546f46d 100644
--- a/hw/9pfs/9p-util.h
+++ b/hw/9pfs/9p-util.h
@@ -37,9 +37,22 @@ static inline int openat_file(int dirfd, const char *name, int flags,
{
int fd, serrno, ret;
+again:
fd = openat(dirfd, name, flags | O_NOFOLLOW | O_NOCTTY | O_NONBLOCK,
mode);
if (fd == -1) {
+ if (errno == EPERM && (flags & O_NOATIME)) {
+ /*
+ * The client passed O_NOATIME but we lack permissions to honor it.
+ * Rather than failing the open, fall back without O_NOATIME. This
+ * doesn't break the semantics on the client side, as the Linux
+ * open(2) man page notes that O_NOATIME "may not be effective on
+ * all filesystems". In particular, NFS and other network
+ * filesystems ignore it entirely.
+ */
+ flags &= ~O_NOATIME;
+ goto again;
+ }
return -1;
}