Re: [PATCH 1/3] config: work around gcc-10 -Wstringop-overflow warning

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> Compiling with gcc-10, -O2, and -fsanitize=undefined results in a
> compiler warning:
>
>   config.c: In function ‘git_config_copy_or_rename_section_in_file’:
>   config.c:3170:17: error: writing 1 byte into a region of size 0 [-Werror=stringop-overflow=]
>    3170 |       output[0] = '\t';
>         |       ~~~~~~~~~~^~~~~~
>   config.c:3076:7: note: at offset -1 to object ‘buf’ with size 1024 declared here
>    3076 |  char buf[1024];
>         |       ^~~
>
> This is a false positive. The interesting lines of code are:
>
>   int i;
>   char *output = buf;
>   ...
>   for (i = 0; buf[i] && isspace(buf[i]); i++)
>           ; /* do nothing */
>   ...
>   int offset;
>   offset = section_name_match(&buf[i], old_name);
>   if (offset > 0) {
>           ...
>           output += offset + i;
>           if (strlen(output) > 0) {
> 		  /*
> 		   * More content means there's
> 		   * a declaration to put on the
> 		   * next line; indent with a
> 		   * tab
> 		   */
> 		  output -= 1;
> 		  output[0] = '\t';
> 	  }
>   }
>
> So we do assign output to buf initially. Later we increment it based on
> "offset" and "i" and then subtract "1" from it. That latter step is what
> the compiler is complaining about; it could lead to going off the left
> side of the array if "output == buf" at the moment of the subtraction.
> For that to be the case, then "offset + i" would have to be 0. But that
> can't happen:
>
>   - we know that "offset" is at least 1, since we're in a conditional
>     block that checks that
>
>   - we know that "i" is not negative, since it started at 0 and only
>     incremented over whitespace
>
> So the sum must be at least 1, and therefore it's OK to subtract one
> from "output".
>
> But that's not quite the whole story. Since "i" is an int, it could in
> theory be possible to overflow to negative (when counting whitespace on
> a very large string). But we know that's impossible because we're
> counting the 1024-byte buffer we just fed to fgets(), so it can never be
> larger than that.
>
> Switching the type of "i" to "unsigned" makes the warning go away, so
> let's do that.
>
> Arguably size_t is an even better type (for this and for the other
> length fields), but switching to it produces a similar but distinct
> warning:
>
>   config.c: In function ‘git_config_copy_or_rename_section_in_file’:
>   config.c:3170:13: error: array subscript -1 is outside array bounds of ‘char[1024]’ [-Werror=array-bounds]
>    3170 |       output[0] = '\t';
>         |       ~~~~~~^~~
>   config.c:3076:7: note: while referencing ‘buf’
>    3076 |  char buf[1024];
>         |       ^~~
>
> If we were to ever switch off of fgets() to strbuf_getline() or similar,
> we'd probably need to use size_t to avoid other overflow problems. But
> for now we know we're safe because of the small fixed size of our
> buffer.
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---

Thanks.  80 lines of informative log message to explain a one liner
was surprisingly pleasnt to read.  Nicely done.

>  config.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/config.c b/config.c
> index 8db9c77098..2b79fe76ad 100644
> --- a/config.c
> +++ b/config.c
> @@ -3115,7 +3115,7 @@ static int git_config_copy_or_rename_section_in_file(const char *config_filename
>  	}
>  
>  	while (fgets(buf, sizeof(buf), config_file)) {
> -		int i;
> +		unsigned i;
>  		int length;
>  		int is_section = 0;
>  		char *output = buf;