Re: [Qemu-devel] [PATCH v2 2/5] linux-user: avoid string truncation warnings in elf field copying

[Laurent Vivier]

On 12/04/2019 14:16, Daniel P. Berrangé wrote:
> In file included from /usr/include/string.h:494,
>                  from include/qemu/osdep.h:101,
>                  from linux-user/elfload.c:2:
> In function ‘strncpy’,
>     inlined from ‘fill_psinfo’ at linux-user/elfload.c:3208:12,
>     inlined from ‘fill_note_info’ at linux-user/elfload.c:3390:5,
>     inlined from ‘elf_core_dump’ at linux-user/elfload.c:3539:9:
> /usr/include/bits/string_fortified.h:106:10: warning: ‘__builtin_strncpy’ 
> specified bound 16 equals destination size [-Wstringop-truncation]
>   106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos 
> (__dest));
>       |          
> ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> We don't require the field to be NUL terminated, so can just
> copy the lower of the string length and the target field size
> using memcpy.
> 
> Signed-off-by: Daniel P. Berrangé <address@hidden>
> ---
>  linux-user/elfload.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index c1a26021f8..caa060f7b7 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -3180,6 +3180,7 @@ static int fill_psinfo(struct target_elf_prpsinfo 
> *psinfo, const TaskState *ts)
>  {
>      char *base_filename;
>      unsigned int i, len;
> +    size_t pathlen;
>  
>      (void) memset(psinfo, 0, sizeof (*psinfo));
>  
> @@ -3201,12 +3202,9 @@ static int fill_psinfo(struct target_elf_prpsinfo 
> *psinfo, const TaskState *ts)
>      psinfo->pr_gid = getgid();
>  
>      base_filename = g_path_get_basename(ts->bprm->filename);
> -    /*
> -     * Using strncpy here is fine: at max-length,
> -     * this field is not NUL-terminated.
> -     */

Keep and update the comment, it explains why we don't need to add the
NUL at the end when MIN() is sizeof(psinfo->pr_fname).

> -    (void) strncpy(psinfo->pr_fname, base_filename,
> -                   sizeof(psinfo->pr_fname));
> +    pathlen = strlen(base_filename) + 1;
> +    pathlen = MIN(pathlen, sizeof(psinfo->pr_fname));
> +    memcpy(psinfo->pr_fname, base_filename, pathlen);
>  
>      g_free(base_filename);
>      bswap_psinfo(psinfo);
> 

Thanks,
Laurent