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Add and use strnlen_a() instead of more complex code #1298

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Add and use strnlen_a() instead of more complex code #1298

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lib/memory/strnlen/: strnlen_a(): Add macro
alejandro-colomar Jul 16, 2025
157e20a
lib/utmp.c: Use countof() for the number of elements of an array
alejandro-colomar Jul 16, 2025
6e8c56f
lib/: Use strnlen_a() instead of its pattern
alejandro-colomar Jul 16, 2025
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2 changes: 2 additions & 0 deletions lib/Makefile.am
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Original file line number Diff line number Diff line change
Expand Up @@ -115,6 +115,8 @@ libshadow_la_SOURCES = \
lockpw.c \
loginprompt.c \
mail.c \
memory/strnlen/strnlen.c \
memory/strnlen/strnlen.h \
motd.c \
myname.c \
nss.c \
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7 changes: 7 additions & 0 deletions lib/memory/strnlen/strnlen.c
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@@ -0,0 +1,7 @@
// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
// SPDX-License-Identifier: BSD-3-Clause


#include "config.h"

#include "memory/strnlen/strnlen.h"
20 changes: 20 additions & 0 deletions lib/memory/strnlen/strnlen.h
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// SPDX-FileCopyrightText: 2025-2026, Alejandro Colomar <alx@kernel.org>
// SPDX-License-Identifier: BSD-3-Clause


#ifndef SHADOW_INCLUDE_LIB_MEMORY_STRNLEN_STRNLEN_H_
#define SHADOW_INCLUDE_LIB_MEMORY_STRNLEN_STRNLEN_H_


#include "config.h"

#include <memory.h>

#include "sizeof.h"


// strnlen_a - nonstring length array
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#define strnlen_a(strn) strnlen(strn, countof(strn))


#endif // include guard
5 changes: 5 additions & 0 deletions lib/string/README
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Expand Up @@ -288,6 +288,11 @@ strsep/ - String separation
Variant of strsep2ls() that allocates the array of strings.
(But the strings themselves are not duplicated.)

strnlen/ - String length

strnlen_a()
Like strnlen(3), but take an array.

@ikerexxe ikerexxe Aug 26, 2026

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Can you add additional information explaining the benefits of this new API?

@alejandro-colomar alejandro-colomar Aug 26, 2026

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The benefits of all _a() macros is that they do the countof() internally, avoiding human mistakes.

@alejandro-colomar alejandro-colomar Aug 26, 2026 •
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In the case of this one, the mistake is less likely than with other string/nonstring APIs, because the others often have 2 parameters, and one can choose the argument wrongly, while in this case it's more obvious. But it still reduces mistakes a little bit. Since we can take it internally reducing code, it has less mistakes.

@alejandro-colomar alejandro-colomar Aug 26, 2026 •
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For example, one thing it does is it makes sure that the input is an array. Nonstrings from utmp are always arrays, so this reduces the chances of accidentally passing a string.

@alejandro-colomar alejandro-colomar Aug 26, 2026

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We have some such information at the top of the README:

General guidelines:
===================

-  If there's an upper-case macro that wraps a function, use the macro
   if possible.  These use macro magic to add safety.

-  If there's a *_a() macro that wraps an API, use the macro if
   possible.  These use countof() to add bounds safety.

...


strftime.h
strftime_a()
Like strftime(3), but takes an array.
4 changes: 2 additions & 2 deletions lib/utmp.c
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Expand Up @@ -30,6 +30,7 @@
#include "alloc/malloc.h"
#include "attr.h"
#include "io/syslog.h"
#include "memory/strnlen/strnlen.h"
#include "sizeof.h"
#include "string/strchr/strnul.h"
#include "string/strcmp/streq.h"
Expand Down Expand Up @@ -301,8 +302,7 @@ prepare_utmp(const char *name, const char *line, const char *host,
strncpy_a(utent->ut_host, hostname);
#endif
#if defined(HAVE_STRUCT_UTMPX_UT_SYSLEN)
utent->ut_syslen = MIN(strlen(hostname),
sizeof(utent->ut_host));
utent->ut_syslen = strnlen_a(utent->ut_host);
#endif
#if defined(HAVE_STRUCT_UTMPX_UT_ADDR) || defined(HAVE_STRUCT_UTMPX_UT_ADDR_V6)
if (getaddrinfo (hostname, NULL, NULL, &info) == 0) {
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