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

[Taylor Blau <me@ttaylorr.com>]

On Tue, Aug 04, 2020 at 09:30:15AM -0700, Junio C Hamano wrote:
> 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.

Agreed, and sorry that this took me so long to read (I thought that I
had read it when you sent it, but apparently not). Your reasoning is
sensible, and I agree that your fix is appropriate.

  Reviewed-by: Taylor Blau <me@ttaylorr.com>

> >  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;

Thanks,
Taylor