[PATCH] 9p: treat read return values of 0 as EOF

[PATCH] 9p: treat read return values of 0 as EOF

[Barret Rhoden]

If your 9p server shares a synthetic file system such as sysfs, the file
size from stat often disagrees with the actual file size.

Treat non-error reads of 0 as EOF, except in the case where the request
was for 0 bytes.

Tested:
	git@github.com:hugelgupf/p9.git, served "/" on 10.0.2.2

	bash-5.3# mount -t 9p -o trans=tcp,port=12345,cache=none 10.0.2.2 /tmp/foo
	bash-5.3# cat /tmp/foo/sys/devices/system/cpu/online
	0-47

Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do reads and caching")
Signed-off-by: Barret Rhoden <brho@google.com>
---
 fs/9p/vfs_addr.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
index 1ac0b3dcc077..df420b061664 100644
--- a/fs/9p/vfs_addr.c
+++ b/fs/9p/vfs_addr.c
@@ -73,6 +73,7 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq)
 	char *target;
 	unsigned long long pos = subreq->start + subreq->transferred;
 	int total = 0, err, len, n;
+	size_t to_read = iov_iter_count(&subreq->io_iter);
 
 	if (S_ISLNK(rreq->inode->i_mode)) {
 		/* p9_client_readlink() must not be called for legacy protocols
@@ -105,6 +106,8 @@ static void v9fs_issue_read(struct netfs_io_subrequest *subreq)
 		__set_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags);
 	if (pos + total >= i_size_read(rreq->inode))
 		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
+	if (!err && !total && to_read)
+		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
 	if (!err && total) {
 		subreq->transferred += total;
 		__set_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
-- 
2.55.0.rc0.799.gd6f94ed593-goog

Re: [PATCH] 9p: treat read return values of 0 as EOF

[Dominique Martinet]

Barret Rhoden wrote on Thu, Jul 02, 2026 at 09:09:32AM +0000:
> If your 9p server shares a synthetic file system such as sysfs, the file
> size from stat often disagrees with the actual file size.
> 
> Treat non-error reads of 0 as EOF, except in the case where the request
> was for 0 bytes.

> Tested:
> 	git@github.com:hugelgupf/p9.git, served "/" on 10.0.2.2
> 
> 	bash-5.3# mount -t 9p -o trans=tcp,port=12345,cache=none 10.0.2.2 /tmp/foo
> 	bash-5.3# cat /tmp/foo/sys/devices/system/cpu/online
> 	0-47

David,
that looks like something that should be handled in netfs - is it right
to do this on the 9p side?

I've just tried with qemu and with cache=none I get ENODATA, with
cache=loose I get the data followed by 4094 zeroes...

Interestingly there also are files with 0 size e.g.
/sys/devices/virtual/net/br0/brforward,
and for this one with cache=none I can read it fine, but with
cache=loose I get an empty file.


Barret,
thanks for the patch, please give us a bit of time to check; I'm
sometimes slow to reply (like for your sending the patch first without
the [PATCH] tag in subject) but I did see it, just hard to find time to
review...

Cheers,
-- 
Dominique

Re: [PATCH] 9p: treat read return values of 0 as EOF

[Barret Rhoden]

On 7/3/26 10:28 AM, Dominique Martinet wrote:
...
> Barret,
> thanks for the patch, please give us a bit of time to check; I'm
> sometimes slow to reply (like for your sending the patch first without
> the [PATCH] tag in subject) but I did see it, just hard to find time to
> review...

no problem - i thought it might have gotten missed due to the lack of 
[PATCH].  my bad on that.

let me know if i can test anything too.

thanks,
barret

Re: [PATCH] 9p: treat read return values of 0 as EOF

[David Howells]

This is kind of a weird situation.  We're caching locally the content of files
aren't really regular files and probably shouldn't be cached.  I'm not sure
what the best way to deal with that is.  I wonder if there's some way to
detect that and mark then non-cacheable.  (Assuming the server can be told not
to even serve them).

Can we detect that the EOF length doesn't match i_size and set a flag to say
"don't cache" in netfs_inode::flags?

David