NetBSD Problem Report #60554

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Date: Fri,  7 Aug 2026 22:19:06 +0000 (UTC)
From: campbell+netbsd@mumble.net
Reply-To: campbell+netbsd@mumble.net
To: gnats-bugs@NetBSD.org
Subject: nfs: integer shift UB in pathconf(_PC_FILESIZEBITS)
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X-From4GNATS: "campbell+netbsd@mumble.net via gnats" <gnats-admin@NetBSD.org>

>Number:         60554
>Category:       kern
>Synopsis:       nfs: integer shift UB in pathconf(_PC_FILESIZEBITS)
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          needs-pullups
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Aug 07 22:20:00 +0000 2026
>Closed-Date:    
>Last-Modified:  Sat Aug 08 00:20:49 +0000 2026
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The NfsBSD Infinite Loop, Inc.
>Environment:
>Description:

	When nmp->nm_maxfilesize is 64 bits long, this logic to compute
	ilog2(nmp->nm_maxfilesize) overflows the shift into undefined
	behaviour:

   3226 			for (l = 0, maxsize = nmp->nm_maxfilesize;
   3227 			    (maxsize >> l) > 0; l++)
   3228 				;

https://nxr.netbsd.org/xref/src/sys/nfs/nfs_vnops.c?r=1.327#3218

	This can be provoked by an nfs server that returns a large
	maxfilesize in the FSINFO request.

>How-To-Repeat:

	1. Set up an NFS server with maxfilesize = 2^64 - 1 (e.g.,
	   FreeBSD serving from ZFS).

	2. Mount it on NetBSD.

	3. Run `getconf GETFILESIZEBITS /mnt'.

>Fix:

	Use ilog2(maxsize) instead of the bespoke loop, taking care to
	handle the fencepost error required by definition of
	_PC_FILESIZEBITS:

     _PC_FILESIZEBITS
             If the maximum size file that could ever exist on the mounted
             file system is maxsize, then the returned value is 2 plus the
             floor of the base 2 logarithm of maxsize.

>Release-Note:

>Audit-Trail:
From: "Taylor R Campbell" <riastradh@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/60554 CVS commit: src/sys/nfs
Date: Sat, 8 Aug 2026 00:08:45 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Sat Aug  8 00:08:45 UTC 2026

 Modified Files:
 	src/sys/nfs: nfs_vnops.c

 Log Message:
 nfs_vnops.c: Sort includes.

 No functional change intended.  Preparation for:

 PR kern/60554: nfs: integer shift UB in pathconf(_PC_FILESIZEBITS)


 To generate a diff of this commit:
 cvs rdiff -u -r1.327 -r1.328 src/sys/nfs/nfs_vnops.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/60554 CVS commit: src/sys/nfs
Date: Sat, 8 Aug 2026 00:09:25 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Sat Aug  8 00:09:25 UTC 2026

 Modified Files:
 	src/sys/nfs: nfs_vnops.c

 Log Message:
 nfs: Avoid UB in integer shift amounts.

 Just use ilog2 explicitly here, instead of writing out a loop with
 shifts and increments to compute it more slowly and buggily.  Note
 the confusing fencepost adjustment.  Clamp _PC_FILESIZEBITS at the
 actual size of off_t since we can't go beyond that.

 PR kern/60554: nfs: integer shift UB in pathconf(_PC_FILESIZEBITS)


 To generate a diff of this commit:
 cvs rdiff -u -r1.328 -r1.329 src/sys/nfs/nfs_vnops.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: Sat, 08 Aug 2026 00:20:49 +0000
State-Changed-Why:
fixed in HEAD, needs pullup-11, pullup-10, pullup-9


>Unformatted:

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