strcat.3, strncat.3: RIP strncat(3)
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Commit Message
Never use this function. Really.
Cc: <pkg-shadow-devel@alioth-lists.debian.net>
Cc: <libc-alpha@sourceware.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
---
Hi!
To shadow-utils readers, I've seen there are a few uses of strncat(3) in
shadow-utils. I'll review my current PR about string handling to also
address this issue.
To glibc readers, please bury this function deep down as if it were
radioactive waste.
Cheers,
Alex
man3/strcat.3 | 75 +++-------------------------------------
man3/strncat.3 | 92 +++++++++++++++++++++++++++++++++++++++++++++++++-
2 files changed, 95 insertions(+), 72 deletions(-)
Comments
On 12/5/22 16:49, Alejandro Colomar wrote:
> Never use this function. Really.
>
> Cc: <pkg-shadow-devel@alioth-lists.debian.net>
> Cc: <libc-alpha@sourceware.org>
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
>
> Hi!
>
> To shadow-utils readers, I've seen there are a few uses of strncat(3) in
> shadow-utils. I'll review my current PR about string handling to also
> address this issue.
>
> To glibc readers, please bury this function deep down as if it were
> radioactive waste.
>
> Cheers,
>
> Alex
>
The rendered version of the new manual page for strncpy(3) is:
strncat(3) Library Functions Manual strncat(3)
NAME
strncat - concatenate two strings
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <string.h>
[[deprecated]]
char *strncat(char dest[restrict strlen(.dest) + strnlen(.n) + 1],
const char src[restrict .n],
size_t n);
DESCRIPTION
Note: Never use this function.
For safe string concatenation, see strlcat(3bsd). For copying
or concatenating a string into a fixed‐length buffer with zero‐
ing of the rest, see stpncpy(3).
strncat() appends at most n characters of src to the end of dst.
It always terminates with a null character the string placed in
dest.
A simple implementation of strncat() might be:
char *
strncat(char *dest, const char *src, size_t n)
{
return memcpy(dest + strlen(dest), src, strnlen(src, n));
}
RETURN VALUE
strncat() returns a pointer to the resulting string dest.
ATTRIBUTES
For an explanation of the terms used in this section, see at‐
tributes(7).
┌─────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├─────────────────────────────────────┼───────────────┼─────────┤
│strncat() │ Thread safety │ MT‐Safe │
└─────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD.
BUGS
All. Seriously, there’s no use case for this function.
It has a very misleading name. This function has no relation‐
ship with strncpy(3).
Since it doesn’t know the size of the destination buffer, this
function can easily write past the end of the array, being an
open door to all kinds of crackers.
SEE ALSO
strcpy(3), string(3)
Linux man‐pages (unreleased) (date) strncat(3)
--
<http://www.alejandro-colomar.es/>
@@ -11,7 +11,7 @@
.\" Improve discussion of strncat().
.TH strcat 3 (date) "Linux man-pages (unreleased)"
.SH NAME
-strcat, strncat \- concatenate two strings
+strcat \- concatenate two strings
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
@@ -20,8 +20,6 @@ .SH SYNOPSIS
.B #include <string.h>
.PP
.BI "char *strcat(char *restrict " dest ", const char *restrict " src );
-.BI "char *strncat(char " dest "[restrict strlen(." dest ") + strnlen(." n ") + 1],"
-.BI " const char " src "[restrict ." n "], size_t " n );
.fi
.SH DESCRIPTION
The
@@ -42,73 +40,10 @@ .SH DESCRIPTION
.I dest
is not large enough, program behavior is unpredictable;
.IR "buffer overruns are a favorite avenue for attacking secure programs" .
-.PP
-The
-.BR strncat ()
-function is similar, except that
-.IP \(bu 3
-it will use at most
-.I n
-bytes from
-.IR src ;
-and
-.IP \(bu
-.I src
-does not need to be null-terminated if it contains
-.I n
-or more bytes.
-.PP
-As with
-.BR strcat (),
-the resulting string in
-.I dest
-is always null-terminated.
-.PP
-If
-.I src
-contains
-.I n
-or more bytes,
-.BR strncat ()
-writes
-.I n+1
-bytes to
-.I dest
-.RI ( n
-from
-.I src
-plus the terminating null byte).
-Therefore, the size of
-.I dest
-must be at least
-.IR "strlen(dest)+n+1" .
-.PP
-A simple implementation of
-.BR strncat ()
-might be:
-.PP
-.in +4n
-.EX
-char *
-strncat(char *dest, const char *src, size_t n)
-{
- size_t dest_len = strlen(dest);
- size_t i;
-
- for (i = 0 ; i < n && src[i] != \(aq\e0\(aq ; i++)
- dest[dest_len + i] = src[i];
- dest[dest_len + i] = \(aq\e0\(aq;
-
- return dest;
-}
-.EE
-.in
.SH RETURN VALUE
The
.BR strcat ()
-and
-.BR strncat ()
-functions return a pointer to the resulting string
+function returns a pointer to the resulting string
.IR dest .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
@@ -175,12 +110,10 @@ .SH NOTES
.SH EXAMPLES
Because
.BR strcat ()
-and
-.BR strncat ()
must find the null byte that terminates the string
.I dest
using a search that starts at the beginning of the string,
-the execution time of these functions
+the execution time of this function
scales according to the length of the string
.IR dest .
This can be demonstrated by running the program below.
@@ -222,6 +155,6 @@ .SH SEE ALSO
.BR memcpy (3),
.BR strcpy (3),
.BR string (3),
-.BR strncpy (3),
+.BR strlcat (3bsd),
.BR wcscat (3),
.BR wcsncat (3)
@@ -1 +1,91 @@
-.so man3/strcat.3
+.\" Copyright 2022 Alejandro Colomar <alx@kernel.org>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH strncat 3 (date) "Linux man-pages (unreleased)"
+.SH NAME
+strncat \- concatenate two strings
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.B #include <string.h>
+.PP
+.B [[deprecated]]
+.BI "char *strncat(char " dest "[restrict strlen(." dest ") + strnlen(." n ") + 1],"
+.BI " const char " src "[restrict ." n ],
+.BI " size_t " n );
+.fi
+.SH DESCRIPTION
+.BI Note: " Never use this function."
+.PP
+For safe string concatenation, see
+.BR strlcat (3bsd).
+For copying or concatenating a string into a fixed-length buffer
+with zeroing of the rest, see
+.BR stpncpy (3).
+.PP
+.BR strncat ()
+appends at most
+.I n
+characters of
+.I src
+to the end of
+.IR dst .
+It always terminates with a null character the string placed in
+.IR dest .
+.PP
+A simple implementation of
+.BR strncat ()
+might be:
+.PP
+.in +4n
+.EX
+char *
+strncat(char *dest, const char *src, size_t n)
+{
+ return memcpy(dest + strlen(dest), src, strnlen(src, n));
+}
+.EE
+.in
+.SH RETURN VALUE
+.BR strncat ()
+returns a pointer to the resulting string
+.IR dest .
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.ad l
+.nh
+.TS
+allbox;
+lbx lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR strncat ()
+T} Thread safety MT-Safe
+.TE
+.hy
+.ad
+.sp 1
+.SH STANDARDS
+POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.
+.SH BUGS
+All.
+Seriously,
+there's no use case for this function.
+.PP
+It has a
+.I very
+misleading name.
+This function has no relationship with
+.BR strncpy (3).
+.PP
+Since it doesn't know the size of the destination buffer,
+this function can easily write past the end of the array,
+being an open door to all kinds of crackers.
+.SH SEE ALSO
+.BR strcpy (3),
+.BR string (3)