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| From: | Martin Sebor |
| Subject: | Re: [-Wstringop-overflow=] strncat(3) |
| Date: | Thu, 15 Dec 2022 13:50:00 -0700 |
| User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 |
On 12/14/22 16:14, Alejandro Colomar via Libc-alpha wrote:
[CC += groff] Hi Andrew, On 12/14/22 23:57, Andrew Pinski wrote:On Wed, Dec 14, 2022 at 2:46 PM Alejandro Colomar via Libc-alpha <libc-alpha@sourceware.org> wrote:Hi,I was rewriting the strncat(3) manual page, and when I tried to compile theexample program, I got a surprise from the compiler. Here goes the page:strncat(3) Library Functions Manual strncat(3)NAMEstrncat - concatenate a null‐padded character sequence into astring LIBRARY Standard C library (libc, -lc) SYNOPSIS #include <string.h>char *strncat(char *restrict dst, const char src[restrict .sz],size_t sz); DESCRIPTIONThis function catenates the input character sequence contained in a null‐padded fixed‐width buffer, into a string at the buffer pointed to by dst. The programmer is responsible for allocating a buffer large enough, that is, strlen(dst) + strnlen(src, sz) + 1.An implementation of this function might be: char *strncat(char *restrict dst, const char *restrict src, size_t sz){ int len; char *end; len = strnlen(src, sz); end = dst + strlen(dst); end = mempcpy(end, src, len); *end = '\0'; return dst; } RETURN VALUE strncat() returns dest. ATTRIBUTES [...] STANDARDS POSIX.1‐2001, POSIX.1‐2008, C89, C99, SVr4, 4.3BSD. CAVEATSThe name of this function is confusing. This function has no re‐lation with strncpy(3).If the destination buffer is not large enough, the behavior is un‐defined. See _FORTIFY_SOURCE in feature_test_macros(7). BUGSThis function can be very inefficient. Read about Shlemiel the painter ⟨https://www.joelonsoftware.com/2001/12/11/back-to-basics/⟩. EXAMPLES #include <stdio.h> #include <stdlib.h> #include <string.h> int main(void) { char buf[BUFSIZ]; size_t len;buf[0] = '\0'; // There’s no ’cpy’ function to this ’cat’.strncat(buf, "Hello ", 6);
There's nothing wrong with this but the two lines above would be more simply coded as one: strcpy(buf, "Hello "); The original code suggests a misunderstanding of strncpy's purpose: that it writes exactly 6 bytes into the destination. That's what the warning points out.
strncat(buf, "world", 42); // Padding null bytes ignored.strncat(buf, "!", 1); len = strlen(buf); printf("[len = %zu]: <%s>\n", len, buf); exit(EXIT_SUCCESS); } SEE ALSO string(3), string_copy(3)Linux man‐pages (unreleased) (date) strncat(3)And when you compile that, you get: $ cc -Wall -Wextra ./strncat.c ./strncat.c: In function ‘main’:./strncat.c:12:12: warning: ‘strncat’ specified bound 6 equals source length[-Wstringop-overflow=] 12 | strncat(buf, "Hello ", 6); | ^~~~~~~~~~~~~~~~~~~~~~~~~./strncat.c:14:12: warning: ‘strncat’ specified bound 1 equals source length[-Wstringop-overflow=] 14 | strncat(buf, "!", 1); | ^~~~~~~~~~~~~~~~~~~~So, what? Where's the problem? This function does exactly that: "take an unterminated character sequence and catenate it to an existing string".
Strncat has historically had two distinct use cases. One of them -- to constrain the amount of data to copy to the space remaining in the destination -- gained popularity with the push to reduce buffer overflow weaknesses in code. Mistakes in these uses gave rise to a whole other class of security bugs, to the extent that CERT felt it necessary to publish the strncpy and strncat best practice. The GCC warning in turn was added to support the CERT guideline. I touch on some of this in a blog post I wrote a few years ago: https://developers.redhat.com/blog/2018/05/24/detecting-string-truncation-with-gcc-8 The specific uses of the function above are contrived (there's no point in calling strncat to append the full string -- strcat will do that more clearly and efficiently) but the general use case -- limiting the amount of copied data to an initial substring of the source sequence -- although valid and originally intended (it's one of the two uses of the function in UNIX v7), is not one that either the guideline or the warning consider. They can only consider one use cases, and they chose the one that was observed behind security bugs. That choice unavoidably leads to some false positives. The expected way to deal with them is to suppress the warning by one of the usual mechanisms (command line option or #pragma GCC diagnostic). Martin
Clang seems to be fine with the code.See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83404 and the background of why the warning was added here:https://www.us-cert.gov/bsi/articles/knowledge/coding-practices/strncpy-and-strncat.This document is bogus, since it's puting strncpy(3) and strncat(3) in the same sack, when they're in reality two completely different beasts. I'll quote below some paragraphs of some new page I'm writing, which will show why.The rationale behind GCC's warning is also fundamentally wrong. Martin was wrong when he claimed that the right call for strncat(3) is the remaining space in the destination.I admit that I didn't know what strncat(3) was useful for, and believed that it was simply a broken-by-design function until very recently (this week, more or less). And to be honest, I still believe it's broken by design; it's just that it can be repurposed for a reasonable new purpose (which I found while digging in groff's source code; that's why the CC).First I'll show an example program that I added to the strncat(3) manual page last week, which is based on the groff code that used it:#include <stdio.h> #include <stdlib.h> #include <string.h> #define nitems(arr) (sizeof((arr)) / sizeof((arr)[0])) int main(void) { char pre[4] = "pre."; char *post = ".post"; char *src = "some_long_body.post"; char dest[100]; dest[0] = '\0'; strncat(dest, pre, nitems(pre)); strncat(dest, src, strlen(src) - strlen(post)); puts(dest); // "pre.some_long_body" exit(EXIT_SUCCESS); } And now I'll quote some text that I'm writing currently for the function: Null‐padded character sequencesFor historic reasons, some standard APIs, such as utmpx(5), use null‐ padded character sequences in fixed‐width buffers. To interface withthem, specialized functions need to be used. To copy strings into them, use stpncpy(3).To copy from an unterminated string within a fixed‐width buffer into a string, ignoring any trailing null bytes in the source fixed‐widthbuffer, you should use strncat(3). [...] stpncpy(3) This function copies the input string into a destination null‐padded character sequence in a fixed‐width buffer. If the destination buffer, limited by its size, isn’t large enough to hold the copy, the resulting character sequence is truncated. Since it creates a character sequence, it doesn’t need to write a terminating null byte. It’s impos‐ sible to distinguish truncation after the call, from a character sequence that just fits the destination buffer; truncation should be detected from the length of the origi‐ nal string. strncpy(3) This function is identical to stpncpy(3) except for the useless return value. stpncpy(3) is a simpler alternative to this function. [...] strncat(3) Do not confuse this function with strncpy(3); they are not related at all. This function catenates the input character sequence con‐ tained in a null‐padded wixed‐width buffer, into a destina‐ tion string. The programmer is responsible for allocating a buffer large enough. The return value is useless. zustr2stp(3) is a faster alternative to this function. An implementation of this function might be: char * strncat(char *restrict dst, const char *restrict src, size_t sz) { int len; char *end; len = strnlen(src, sz); end = dst + strlen(dst); end = mempcpy(end, src, len); *end = '\0'; return dst; } Cheers, AlexThanks, Andrew PinskiCheers, Alex -- <http://www.alejandro-colomar.es/>
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