[Bug c/102875] New: __builtin_strncpy output may be truncated copying bytes from a string of length

[Bug c/102875] New: __builtin_strncpy output may be truncated copying bytes from a string of length

[johnnymarler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102875

            Bug ID: 102875
           Summary: __builtin_strncpy output may be truncated copying
                    bytes from a string of length
           Product: gcc
           Version: 9.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johnnymarler at gmail dot com
  Target Milestone: ---

GCC Version: 9.2.0
System: NixOS

$ gcc -v
Using built-in specs.
COLLECT_GCC=/nix/store/b3zsk4ihlpiimv3vff86bb5bxghgdzb9-gcc-9.2.0/bin/gcc
COLLECT_LTO_WRAPPER=/nix/store/b3zsk4ihlpiimv3vff86bb5bxghgdzb9-gcc-9.2.0/libexec/gcc/x86_64-unknown-linux-gnu/9.2.0/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: 
Thread model: posix
gcc version 9.2.0 (GCC) 

How to reproduce:

wtf.c
------------------------------
#include <string.h>

// value of 2 works???
//#define COPY_COUNT 2
// values >3 don't work
#define COPY_COUNT 3

typedef struct {
  // NOTE: these padding values don't fix it
  //int something_to_pad_between_elements_1;
  char array[100];
  //int something_to_pad_between_elements_2;
} StringBuf;

int main()
{
  StringBuf a[COPY_COUNT];
  StringBuf b[COPY_COUNT];

  for (unsigned i = 0; i < COPY_COUNT; i++) {
    strncpy(a[i].array, b[i].array, 100);
  }
}
------------------------------

$ gcc -Wall wtf.c
In file included from
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/string.h:494,
                 from wtf.c:1:
In function ‘strncpy’,
    inlined from ‘main’ at wtf.c:21:5:
/nix/store/fwpn2f7a4iqszyydw7ag61zlnp6xk5d3-glibc-2.30-dev/include/bits/string_fortified.h:106:10:
warning: ‘__builtin_strncpy’ output may be truncated copying 100 bytes from a
string of length 299 [-Wstringop-truncation]
  106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos
(__dest));
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Expected Results:

This example is expected to compile with no issues.

Actual Results:

The compiler erroneously warns about a truncated strcpy when no such truncation
occurs.

[Bug c/102875] __builtin_strncpy output may be truncated copying bytes from a string of length

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102875

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
      Known to fail|                            |10.3.0, 11.2.0, 9.4.1
   Last reconfirmed|                            |2021-10-21
           Keywords|                            |diagnostic

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed with the branch head, but you need -O2 -Wall to reproduce.  It seems
fixed in GCC 10 and GCC 11.

[Bug c/102875] __builtin_strncpy output may be truncated copying bytes from a string of length

[johnnymarler at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102875

Jonathan Marler <johnnymarler at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|NEW                         |RESOLVED

--- Comment #2 from Jonathan Marler <johnnymarler at gmail dot com> ---
I confirmed this is fixed in 10.3.0.