bad fixit for Wstrncat-size in macro


Bugzilla Link	17200
Version	trunk
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@efriedma-quic

Extended Description

Example:

nlewycky@ducttape:~$ llvm/Debug+Asserts/bin/clang -fsyntax-only strncat-macro.c
In file included from strncat-macro.c:1:
./strncat-hdr.h:6:24: warning: the value of the size argument in 'strncat' is
      too large, might lead to a buffer overflow [-Wstrncat-size]
    (strncat((A), (B), sizeof(A)-strlen(A)), *((A)+sizeof(A)-1) = '\0')
                       ^~~~~~~~~~~~~~~~~~~
./strncat-hdr.h:5:24: note: change the argument to be the free space in the
      destination buffer minus the terminating null byte
    (strncat((A), (B), sizeof(A)-strlen(A)), *((A)+sizeof(A)-1) = '\0')
                       ^~~~~~~~~~~~~~~~~~~
                       sizeof(dtString) - strlen(dtString) - 1
1 warning generated.

Of course writing in "dtString" for macro argument "A" isn't right. Here's the two-file testcase: nlewycky@ducttape:~$ cat strncat-macro.c

#include "strncat-hdr.h"

void test() {
  char dtString[64];
  char usecString[64];
  STRSCAT(dtString, usecString);
}

nlewycky@ducttape:~$ cat strncat-hdr.h

#define strncat(dest, src, n) __builtin_strncat (dest, src, n)
unsigned long strlen(const char *s);

#define STRSCAT(A,B) \
    (strncat((A), (B), sizeof(A)-strlen(A)), *((A)+sizeof(A)-1) = '\0'

llvm / llvm-project

bad fixit for Wstrncat-size in macro #17574

Extended Description