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Date:   Mon, 18 Apr 2022 07:17:53 +0900
From:   asmadeus@codewreck.org
To:     David Howells <dhowells@redhat.com>,
        Christian Schoenebeck <linux_oss@crudebyte.com>
Cc:     David Kahurani <k.kahurani@gmail.com>, davem@davemloft.net,
        ericvh@gmail.com, kuba@kernel.org, linux-kernel@vger.kernel.org,
        lucho@ionkov.net, netdev@vger.kernel.org,
        v9fs-developer@lists.sourceforge.net, Greg Kurz <groug@kaod.org>
Subject: 9p EBADF with cache enabled (Was: 9p fs-cache tests/benchmark (was:
 9p fscache Duplicate cookie detected))
Message-ID: <YlySEa6QGmIHlrdG@codewreck.org>
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 <YlyFEuTY7tASl8aY@codewreck.org>
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(fixed the subject again and promoted David Howells to To, please read
the previous couple of mails when you have time)

asmadeus@codewreck.org wrote on Mon, Apr 18, 2022 at 06:22:26AM +0900:
> Christian Schoenebeck wrote on Sun, Apr 17, 2022 at 03:52:43PM +0200:
> > > From the looks of it, write fails in v9fs_write_begin, which itself
> > > fails because it tries to read first on a file that was open with
> > > O_WRONLY|O_CREAT|O_APPEND.
> > > Since this is an append the read is necessary to populate the local page
> > > cache when writing, and we're careful that the writeback fid is open in
> > > write, but not about read...

BTW now this is understood here's a much simpler reproducer:

---append.c----
#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
	if (argc < 2)
		return 1;
	int fd = open(argv[1], O_WRONLY|O_APPEND);
	if (fd < 0)
		return 1;
	if (write(fd, "test\n", 5) < 0)
		return 1;
	return 0;
}
---

---
echo foo > foo
echo 3 > /proc/sys/vm/drop_caches
strace ./append foo
...
openat(AT_FDCWD, "foo", O_WRONLY|O_APPEND) = 3
write(3, "test\n", 5)                   = -1 EBADF (Bad file descriptor)
---

at 9p client level:
----
9pnet: (00000460) >>> TWALK fids 1,2 nwname 1d wname[0] t
9pnet: (00000460) >>> size=20 type: 110 tag: 0
9pnet: (00000460) <<< size=22 type: 111 tag: 0
9pnet: (00000460) <<< RWALK nwqid 1:
9pnet: (00000460) <<<     [0] 0.6e672b.6289a895
9pnet: (00000460) >>> TGETATTR fid 2, request_mask 6143
9pnet: (00000460) >>> size=19 type: 24 tag: 0
9pnet: (00000460) <<< size=160 type: 25 tag: 0
9pnet: (00000460) <<< RGETATTR st_result_mask=6143
<<< qid=0.6e672b.6289a895
<<< st_mode=000081ed st_nlink=1
<<< st_uid=1000 st_gid=100
<<< st_rdev=0 st_size=d538 st_blksize=126976 st_blocks=112
<<< st_atime_sec=1650233493 st_atime_nsec=697920121
<<< st_mtime_sec=1650233493 st_mtime_nsec=19911120
<<< st_ctime_sec=1650233493 st_ctime_nsec=19911120
<<< st_btime_sec=0 st_btime_nsec=0
<<< st_gen=0 st_data_version=0
9pnet: (00000460) >>> TWALK fids 2,3 nwname 0d wname[0] (null)
9pnet: (00000460) >>> size=17 type: 110 tag: 0
9pnet: (00000460) <<< size=9 type: 111 tag: 0
9pnet: (00000460) <<< RWALK nwqid 0:
9pnet: (00000460) >>> TLOPEN fid 3 mode 32768
9pnet: (00000460) >>> size=15 type: 12 tag: 0
9pnet: (00000460) <<< size=24 type: 13 tag: 0
9pnet: (00000460) <<< RLOPEN qid 0.6e672b.6289a895 iounit 1f000
9pnet: (00000460) >>> TREAD fid 3 offset 0 8192
9pnet: (00000460) >>> size=23 type: 116 tag: 0
9pnet: (00000460) <<< size=8203 type: 117 tag: 0
9pnet: (00000460) <<< RREAD count 8192
9pnet: (00000460) >>> TREAD fid 3 offset 8192 16384
9pnet: (00000460) >>> size=23 type: 116 tag: 0
9pnet: (00000460) <<< size=16395 type: 117 tag: 0
9pnet: (00000460) <<< RREAD count 16384
9pnet: (00000460) >>> TXATTRWALK file_fid 2, attr_fid 4 name security.capability
9pnet: (00000460) >>> size=36 type: 30 tag: 0
9pnet: (00000460) <<< size=11 type: 7 tag: 0
9pnet: (00000460) <<< RLERROR (-95)
9pnet: (00000460) >>> TREAD fid 3 offset 24576 30008
9pnet: (00000460) >>> size=23 type: 116 tag: 0
9pnet: (00000460) <<< size=30019 type: 117 tag: 0
9pnet: (00000460) <<< RREAD count 30008
9pnet: (00000460) >>> TWALK fids 1,4 nwname 1d wname[0] foo
9pnet: (00000460) >>> size=22 type: 110 tag: 0
9pnet: (00000460) <<< size=22 type: 111 tag: 0
9pnet: (00000460) <<< RWALK nwqid 1:
9pnet: (00000460) <<<     [0] 0.6e66f9.625c86a5
9pnet: (00000460) >>> TGETATTR fid 4, request_mask 6143
9pnet: (00000460) >>> size=19 type: 24 tag: 0
9pnet: (00000460) <<< size=160 type: 25 tag: 0
9pnet: (00000460) <<< RGETATTR st_result_mask=6143
<<< qid=0.6e66f9.625c86a5
<<< st_mode=000081a4 st_nlink=1
<<< st_uid=0 st_gid=0
<<< st_rdev=0 st_size=9 st_blksize=126976 st_blocks=8
<<< st_atime_sec=1650233249 st_atime_nsec=226674419
<<< st_mtime_sec=1650233253 st_mtime_nsec=226727529
<<< st_ctime_sec=1650233253 st_ctime_nsec=226727529
<<< st_btime_sec=0 st_btime_nsec=0
<<< st_gen=0 st_data_version=0
9pnet: (00000460) >>> TWALK fids 4,5 nwname 0d wname[0] (null)
9pnet: (00000460) >>> size=17 type: 110 tag: 0
9pnet: (00000460) <<< size=9 type: 111 tag: 0
9pnet: (00000460) <<< RWALK nwqid 0:
9pnet: (00000460) >>> TLOPEN fid 5 mode 33793
9pnet: (00000460) >>> size=15 type: 12 tag: 0
9pnet: (00000460) <<< size=24 type: 13 tag: 0
9pnet: (00000460) <<< RLOPEN qid 0.6e66f9.625c86a5 iounit 1f000
9pnet: (00000460) >>> TWALK fids 4,6 nwname 0d wname[0] (null)
9pnet: (00000460) >>> size=17 type: 110 tag: 0
9pnet: (00000460) <<< size=9 type: 111 tag: 0
9pnet: (00000460) <<< RWALK nwqid 0:
9pnet: (00000460) >>> TLOPEN fid 6 mode 2
9pnet: (00000460) >>> size=15 type: 12 tag: 0
9pnet: (00000460) <<< size=24 type: 13 tag: 0
9pnet: (00000460) <<< RLOPEN qid 0.6e66f9.625c86a5 iounit 1f000
9pnet: (00000460) >>> TXATTRWALK file_fid 4, attr_fid 7 name security.capability
9pnet: (00000460) >>> size=36 type: 30 tag: 0
9pnet: (00000460) <<< size=11 type: 7 tag: 0
9pnet: (00000460) <<< RLERROR (-95)
9pnet: (00000460) >>> TREAD fid 5 offset 0 9
9pnet: (00000460) >>> size=23 type: 116 tag: 0
9pnet: (00000460) <<< size=11 type: 7 tag: 0
9pnet: (00000460) <<< RLERROR (-9)
9pnet: (00000460) >>> TCLUNK fid 5 (try 0)
9pnet: (00000460) >>> size=11 type: 120 tag: 0
9pnet: (00000460) <<< size=7 type: 121 tag: 0
9pnet: (00000460) <<< RCLUNK fid 5
9pnet: (00000460) >>> TCLUNK fid 3 (try 0)
9pnet: (00000460) >>> size=11 type: 120 tag: 0
9pnet: (00000460) <<< size=7 type: 121 tag: 0
9pnet: (00000460) <<< RCLUNK fid 3
-------

-- 
Dominique