NetBSD Problem Report #60789

From www@netbsd.org  Fri Sep 25 15:00:35 2026
Return-Path: <www@netbsd.org>
Received: from mail.netbsd.org (mail.netbsd.org [199.233.217.200])
	(using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits)
	 key-exchange X25519 server-signature RSA-PSS (2048 bits)
	 client-signature RSA-PSS (2048 bits))
	(Client CN "mail.netbsd.org", Issuer "YR1" (not verified))
	by mollari.NetBSD.org (Postfix) with ESMTPS id 343121A923C
	for <gnats-bugs@gnats.NetBSD.org>; Fri, 25 Sep 2026 15:00:35 +0000 (UTC)
Message-Id: <20260925150034.1AC5B1A923E@mollari.NetBSD.org>
Date: Fri, 25 Sep 2026 15:00:34 +0000 (UTC)
From: campbell+netbsd@mumble.net
Reply-To: campbell+netbsd@mumble.net
To: gnats-bugs@NetBSD.org
Subject: read on fifo without writer may block
X-Send-Pr-Version: www-1.0
X-From4GNATS: "campbell+netbsd@mumble.net via gnats" <gnats-admin@NetBSD.org>

>Number:         60789
>Category:       kern
>Synopsis:       read on fifo without writer may block
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          needs-pullups
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Sep 25 15:05:00 +0000 2026
>Closed-Date:    
>Last-Modified:  Fri Oct 02 03:55:02 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, ...
>Organization:
End-of-fifo Blocks and Pipes LLC
>Environment:
>Description:

	POSIX sez:

	> When attempting to read from an empty pipe or FIFO:
	>
	> - If no process has the pipe open for writing, read() shall
	>   return 0 to indicate end-of-file.

	https://pubs.opengroup.org/onlinepubs/9799919799/functions/read.html

	And that sometimes happens.  But sometimes, on a named pipe,
	read() blocks instead.

>How-To-Repeat:

	barge into a GNU coreutils bug discussion certain that NetBSD
	is right _this_ time and walk away with tail between legs

>Fix:

	Yes, please!

>Release-Note:

>Audit-Trail:
From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/60789 CVS commit: src/tests/lib/libc/sys
Date: Fri, 25 Sep 2026 15:57:31 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Fri Sep 25 15:57:31 UTC 2026

 Modified Files:
 	src/tests/lib/libc/sys: t_mkfifo.c

 Log Message:
 t_mkfifo: New test for consistent read EOF rule.

 PR kern/60789: read on fifo without writer may block


 To generate a diff of this commit:
 cvs rdiff -u -r1.7 -r1.8 src/tests/lib/libc/sys/t_mkfifo.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

State-Changed-From-To: open->needs-pullups
State-Changed-By: riastradh@NetBSD.org
State-Changed-When: Thu, 01 Oct 2026 22:47:58 +0000
State-Changed-Why:
fixed in HEAD, needs pullup-11, pullup-10 after the testbed runs


From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/60789 CVS commit: src
Date: Thu, 1 Oct 2026 22:45:45 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Thu Oct  1 22:45:45 UTC 2026

 Modified Files:
 	src/sys/miscfs/fifofs: fifo_vnops.c
 	src/tests/lib/libc/sys: t_mkfifo.c

 Log Message:
 fifo: Fix EOF reporting in various cases.

 According to POSIX:

 > When attempting to read from an empty pipe or FIFO:
 >
 >     If no process has the pipe open for writing, read() shall
 >     return 0 to indicate end-of-file.

 So:

 1. When there are no writers, after open(O_RDONLY|O_NONBLOCK),
    blocking reads should immediately report EOF instead of blocking,
    and should continue to immediately report EOF on repeated reads.

    Previously, they would simply block, because the wrong socket was
    initialized with SS_CANTRCVMORE -- though curiously, nonblocking
    reads would consistently report EOF.

 2. When the last writer has closed, blocking reads should immediately
    report report EOF instead of blocking, and should continue to
    immediately report EOF on repeated reads.

    Previously, the _first_ such read would report EOF, and
    _subsequent_ reads would block because we cleared SS_CANTRCVMORE
    on the first read.

 The old behaviour dates back to 1990 when the fifo_vnops.c code was
 first added to the UCB CSRG BSD code in SCCS (without any commit
 message):

 https://github.com/robohack/ucb-csrg-bsd/commit/1036021a3155e1972e24dae4115eaf0167c4a3fb

 This change simply applies FreeBSD's fifo_vnops.c rev. 1.39 by Bruce
 Evans from 1997(!):

 https://cgit.freebsd.org/src/commit/?id=80987b7a3b0ba57a094d9b5c42af439d5f94fe8f

 commit 80987b7a3b0ba57a094d9b5c42af439d5f94fe8f
 Author: Bruce Evans <bde@FreeBSD.org>
 Date:   Sat Dec 13 13:49:59 1997 +0000

     Fixed EOF handing.

     1. SS_CANTRCVMORE was initially set on the wrong socket, so reads
     when there has never been a writer on the socket did not return 0.
     Note that such reads are only possible if the fifo was opened in
     (O_RDONLY | O_NONBLOCK) mode.

     2. SS_CANTSENDMORE was initially set on the wrong socket, but this
     was harmless because the wrong socket is never sent from and there
     is no need to set the flag initially on the right socket (since open
     in (O_WRONLY | O_NONBLOCK) mode fails if there is no reader...).

     3. SS_CANTRCVMORE was cleared when read() returns.  This broke the
     case where read() returns 0 - subsequent reads are supposed to
     return 0 until a writer appears.  There is no need to clear the
     flag when read() returns, since it is cleared correctly when a
     writer appears.

 PR kern/60789: read on fifo without writer may block


 To generate a diff of this commit:
 cvs rdiff -u -r1.92 -r1.93 src/sys/miscfs/fifofs/fifo_vnops.c
 cvs rdiff -u -r1.8 -r1.9 src/tests/lib/libc/sys/t_mkfifo.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/60789 CVS commit: src
Date: Fri, 2 Oct 2026 03:53:00 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Fri Oct  2 03:53:00 UTC 2026

 Modified Files:
 	src/sys/miscfs/fifofs: fifo_vnops.c
 	src/tests/lib/libc/sys: t_poll.c

 Log Message:
 fifo: Followups to fix previous fifo_vnops.c change.

 1. Fix open so with O_RDONLY and not O_NONBLOCK, it will block as
    POSIX specifies until there are writers -- not until there are
    writers _or_ a read is possible without blocking (which is always
    immediately true because read will immediately report EOF).

    This was a regression in the previous change which I failed to
    catch because I committed the change a few days ago to hg and
    forgot that I hadn't yet run all the applicable tests when I
    exported it to CVS today -- I had only done the background
    research and thought about the semantics.

 2. Clear xfail on various tests that were fixed by the change.

 PR kern/59056: poll POLLHUP bugs
 PR kern/60789: read on fifo without writer may block


 To generate a diff of this commit:
 cvs rdiff -u -r1.93 -r1.94 src/sys/miscfs/fifofs/fifo_vnops.c
 cvs rdiff -u -r1.17 -r1.18 src/tests/lib/libc/sys/t_poll.c

 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.

>Unformatted:

Home	PR Database Search