Re: [PATCH] fix hang in grep -F for empty string search

[Jim Meyering]

Paolo Bonzini wrote:
...
> Subject: [PATCH 3/5] grep: fix grep -F against empty string
>
> * src/searchutils.c (is_mb_middle): Do not return true for empty matches
> when p == buf.
> ---
>  NEWS              |    3 +++
>  src/searchutils.c |    7 ++++++-
>  2 files changed, 9 insertions(+), 1 deletions(-)
>
> diff --git a/NEWS b/NEWS
> index e822ea1..eb94184 100644
> --- a/NEWS
> +++ b/NEWS
> @@ -4,6 +4,9 @@ GNU grep NEWS                                    -*- outline 
> -*-
>
>  ** Bug fixes
>
> +  Searching with grep -F for an empty string in a multibyte locale
> +  would hang grep. [bug introduced in 2.6.2]
> +
>    PCRE support is once again detected on systems with <pcre/pcre.h>
>    [bug introduced in 2.6.2]
>
> diff --git a/src/searchutils.c b/src/searchutils.c
> index 8c34e31..b04f36f 100644
> --- a/src/searchutils.c
> +++ b/src/searchutils.c
> @@ -142,6 +142,11 @@ is_mb_middle (const char **good, const char *buf, const 
> char *end,
>      }
>
>    *good = prev;
> -  return p > buf || match_len < mbrlen (p, end - p, &cur_state);
> +
> +  if (p > buf)
> +    return true;
> +
> +  /* P == BUF here.  */
> +  return match_len > 0 && match_len < mbrlen (p, end - p, &cur_state);

This is the correct fix, but please rewrite that first comparison
to perform the equivalent test using "<" rather than ">".
That makes it easier to see that we're performing a range test:

    return 0 < match_len && match_len < mbrlen (p, end - p, &cur_state);

Thanks for writing the NEWS entry, too.