Re: [PATCH 1/6] seq_file: add seq_read_iter

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Nov 13, 2020 at 08:01:24PM -0700, Nathan Chancellor wrote:
> Sure thing, it does trigger.
> 
> [    0.235058] ------------[ cut here ]------------
> [    0.235062] WARNING: CPU: 15 PID: 237 at fs/seq_file.c:176 seq_read_iter+0x3b3/0x3f0
> [    0.235064] CPU: 15 PID: 237 Comm: localhost Not tainted 5.10.0-rc2-microsoft-cbl-00002-g6a9f696d1627-dirty #15
> [    0.235065] RIP: 0010:seq_read_iter+0x3b3/0x3f0
> [    0.235066] Code: ba 01 00 00 00 e8 6d d2 fc ff 4c 89 e7 48 89 ee 48 8b 54 24 10 e8 ad 8b 45 00 49 01 c5 48 29 43 18 48 89 43 10 e9 61 fe ff ff <0f> 0b e9 6f fc ff ff 0f 0b 45 31 ed e9 0d fd ff ff 48 c7 43 18 00
> [    0.235067] RSP: 0018:ffff9c774063bd08 EFLAGS: 00010246
> [    0.235068] RAX: ffff91a77ac01f00 RBX: ffff91a50133c348 RCX: 0000000000000001
> [    0.235069] RDX: ffff9c774063bdb8 RSI: ffff9c774063bd60 RDI: ffff9c774063bd88
> [    0.235069] RBP: 0000000000000000 R08: 0000000000000000 R09: ffff91a50058b768
> [    0.235070] R10: ffff91a7f79f0000 R11: ffffffffbc2c2030 R12: ffff9c774063bd88
> [    0.235070] R13: ffff9c774063bd60 R14: ffff9c774063be48 R15: ffff91a77af58900
> [    0.235072] FS:  000000000029c800(0000) GS:ffff91a7f7bc0000(0000) knlGS:0000000000000000
> [    0.235073] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    0.235073] CR2: 00007ab6c1fabad0 CR3: 000000037a004000 CR4: 0000000000350ea0
> [    0.235074] Call Trace:
> [    0.235077]  seq_read+0x127/0x150
> [    0.235078]  proc_reg_read+0x42/0xa0
> [    0.235080]  do_iter_read+0x14c/0x1e0
> [    0.235081]  do_readv+0x18d/0x240
> [    0.235083]  do_syscall_64+0x33/0x70
> [    0.235085]  entry_SYSCALL_64_after_hwframe+0x44/0xa9

*blink*

	Lovely...  For one thing, it did *not* go through
proc_reg_read_iter().  For another, it has hit proc_reg_read() with
zero length, which must've been an iovec with zero ->iov_len in
readv(2) arguments.  I wonder if we should use that kind of
pathology (readv() with zero-length segment in the middle of
iovec array) for regression tests...

	OK...  First of all, since that kind of crap can happen,
let's do this (incremental to be folded); then (and that's
a separate patch) we ought to switch the proc_ops with ->proc_read
equal to seq_read to ->proc_read_iter = seq_read_iter, so that
those guys would not mess with seq_read() wrapper at all.

	Finally, is there any point having do_loop_readv_writev()
call any methods for zero-length segments?

	In any case, the following should be folded into
"fix return values of seq_read_iter()"; could you check if that
fixes the problem you are seeing?

diff --git a/fs/seq_file.c b/fs/seq_file.c
index 07b33c1f34a9..e66d6b8bae23 100644
--- a/fs/seq_file.c
+++ b/fs/seq_file.c
@@ -211,9 +211,9 @@ ssize_t seq_read_iter(struct kiocb *iocb, struct iov_iter *iter)
 		m->count -= n;
 		m->from += n;
 		copied += n;
-		if (!iov_iter_count(iter) || m->count)
-			goto Done;
 	}
+	if (m->count || !iov_iter_count(iter))
+		goto Done;
 	/* we need at least one record in buffer */
 	m->from = 0;
 	p = m->op->start(m, &m->index);