NetBSD Problem Report #59540

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Date: Mon, 21 Jul 2025 18:58:49 +0000 (UTC)
From: rwhitlock22@gmail.com
Reply-To: rwhitlock22@gmail.com
To: gnats-bugs@NetBSD.org
Subject: strlcpy(3) has an off-by-one in the manual page
X-Send-Pr-Version: www-1.0

>Number:         59540
>Category:       lib
>Synopsis:       strlcpy(3) has an off-by-one in the manual page
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    rillig
>State:          closed
>Class:          doc-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Jul 21 19:00:00 +0000 2025
>Closed-Date:    Wed Mar 04 19:38:58 +0000 2026
>Last-Modified:  Wed Mar 04 19:38:58 +0000 2026
>Originator:     Robert Whitlock
>Release:        NetBSD current, 10.99.14, July 11, 2025
>Organization:
>Environment:
see above
>Description:
strlcpy(3) claims that strlcpy leaves bytes past dst[strlen(src) + 1] uninitialized, when it would be more precise to say that it leaves bytes past dst[strlen(src)] uninitialized. This can be easily seen by considering the case where src is the empty string "". Then strlen(src) == 0 and dst[strlen(src) + 1] == dst[1], which is one past the terminating NUL byte that is there when strlcpy has finished. The first byte past dst[1] is then the second byte past the NUL terminating byte.

The patch also includes a minor formatting fix.
>How-To-Repeat:
man strlcpy
>Fix:
Index: strlcpy.3
===================================================================
RCS file: /cvsroot/src/lib/libc/string/strlcpy.3,v
retrieving revision 1.25
diff -u -r1.25 strlcpy.3
--- strlcpy.3	2 Apr 2025 00:50:19 -0000	1.25
+++ strlcpy.3	21 Jul 2025 18:18:15 -0000
@@ -79,7 +79,7 @@
 then the
 .Fn strlcat
 function returns
-.Fa size + Fn strlen src
+.Fa size Li + Fn strlen src
 without writing anything to
 .Fa dst .
 .Pp
@@ -146,11 +146,11 @@
 \(em
 .Fn strlcpy
 leaves bytes past
-.Fa dst Ns Li "[" Fn strlen src Li "+ 1" Ns Li "]"
+.Fa dst Ns Li "[" Fn strlen src Ns Li "]"
 uninitialized, and
 .Fn strlcat
 leaves bytes past
-.Fa dst Ns Li "[" Fn strlen dst Li + Fn strlen src Li + 1 Ns Li "]"
+.Fa dst Ns Li "[" Fn strlen dst Li + Fn strlen src Ns Li "]"
 uninitialized.
 This can lead to security vulnerabilities such as leaking secrets from
 uninitialized stack or heap buffers.

>Release-Note:

>Audit-Trail:

Responsible-Changed-From-To: lib-bug-people->rillig
Responsible-Changed-By: rillig@NetBSD.org
Responsible-Changed-When: Sun, 04 Jan 2026 14:33:33 +0000
Responsible-Changed-Why:
I'll handle it.


State-Changed-From-To: open->feedback
State-Changed-By: rillig@NetBSD.org
State-Changed-When: Sun, 04 Jan 2026 14:33:33 +0000
State-Changed-Why:

What about "dst + strlen(src) + 1" instead of the brackets notation?
That would keep the "+ 1" to emphasize the terminating null byte.


State-Changed-From-To: feedback->analyzed
State-Changed-By: rillig@NetBSD.org
State-Changed-When: Sun, 04 Jan 2026 15:37:07 +0000
State-Changed-Why:
State "feedback" is only for finished work.


From: "Roland Illig" <rillig@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/59540 CVS commit: src/lib/libc/string
Date: Sun, 4 Jan 2026 17:01:30 +0000

 Module Name:	src
 Committed By:	rillig
 Date:		Sun Jan  4 17:01:30 UTC 2026

 Modified Files:
 	src/lib/libc/string: strlcpy.3

 Log Message:
 strlcpy.3: fix off-by-one error (since 2023-08-11)

 Patch provided by Robert Whitlock in PR lib/59540.


 To generate a diff of this commit:
 cvs rdiff -u -r1.25 -r1.26 src/lib/libc/string/strlcpy.3

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

From: "Roland Illig" <rillig@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/59540 CVS commit: src/lib/libc/string
Date: Sun, 4 Jan 2026 17:16:39 +0000

 Module Name:	src
 Committed By:	rillig
 Date:		Sun Jan  4 17:16:39 UTC 2026

 Modified Files:
 	src/lib/libc/string: strlcpy.3

 Log Message:
 strlcpy.3: update date for the recent change

 Also needs to go to pull-ups for PR lib/59540.


 To generate a diff of this commit:
 cvs rdiff -u -r1.26 -r1.27 src/lib/libc/string/strlcpy.3

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

From: "Martin Husemann" <martin@netbsd.org>
To: gnats-bugs@gnats.NetBSD.org
Cc: 
Subject: PR/59540 CVS commit: [netbsd-11] src/lib/libc/string
Date: Wed, 4 Mar 2026 19:18:11 +0000

 Module Name:	src
 Committed By:	martin
 Date:		Wed Mar  4 19:18:11 UTC 2026

 Modified Files:
 	src/lib/libc/string [netbsd-11]: strlcpy.3

 Log Message:
 Pull up following revision(s) (requested by rillig in ticket #190):

 	lib/libc/string/strlcpy.3: revision 1.26
 	lib/libc/string/strlcpy.3: revision 1.27

 strlcpy.3: fix off-by-one error (since 2023-08-11)

 Patch provided by Robert Whitlock in PR lib/59540.

 strlcpy.3: update date for the recent change

 Also needs to go to pull-ups for PR lib/59540.


 To generate a diff of this commit:
 cvs rdiff -u -r1.25 -r1.25.2.1 src/lib/libc/string/strlcpy.3

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

State-Changed-From-To: analyzed->closed
State-Changed-By: rillig@NetBSD.org
State-Changed-When: Wed, 04 Mar 2026 19:38:58 +0000
State-Changed-Why:
Done and pulled up to the netbsd-11 branch.


>Unformatted:

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