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[PATCH 11/77] 9pfs: local: ignore O_NOATIME if we don't have permissions
From: |
Michael Roth |
Subject: |
[PATCH 11/77] 9pfs: local: ignore O_NOATIME if we don't have permissions |
Date: |
Thu, 3 Sep 2020 15:58:29 -0500 |
From: Omar Sandoval <osandov@fb.com>
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>
(cherry picked from commit a5804fcf7b22fc7d1f9ec794dd284c7d504bd16b)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
---
hw/9pfs/9p-util.h | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
index 79ed6b233e..546f46dc7d 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;
}
--
2.17.1
- [PATCH 00/77] Patch Round-up for stable 5.0.1, freeze on 2020-09-10, Michael Roth, 2020/09/03
- [PATCH 09/77] virtio-balloon: unref the iothread when unrealizing, Michael Roth, 2020/09/03
- [PATCH 11/77] 9pfs: local: ignore O_NOATIME if we don't have permissions,
Michael Roth <=
- [PATCH 10/77] block: Call attention to truncation of long NBD exports, Michael Roth, 2020/09/03
- [PATCH 13/77] xen-9pfs: Fix log messages of reply errors, Michael Roth, 2020/09/03
- [PATCH 12/77] 9pfs: include linux/limits.h for XATTR_SIZE_MAX, Michael Roth, 2020/09/03
- [PATCH 15/77] xen/9pfs: yield when there isn't enough room on the ring, Michael Roth, 2020/09/03
- [PATCH 14/77] Revert "9p: init_in_iov_from_pdu can truncate the size", Michael Roth, 2020/09/03
- [PATCH 16/77] ati-vga: check mm_index before recursive call (CVE-2020-13800), Michael Roth, 2020/09/03
- [PATCH 17/77] es1370: check total frame count against current frame, Michael Roth, 2020/09/03
- [PATCH 18/77] Fix tulip breakage, Michael Roth, 2020/09/03
- [PATCH 19/77] iotests/283: Use consistent size for source and target, Michael Roth, 2020/09/03