[PATCH v4 0/3] Fix a segfault caused by regexec() being called on mmap()ed data

[Cc:ing Benjamin Kramer & René Scharfe because they both worked on the REG_STARTEND code in grep.c that I replace in this iteration of the patch series]

This patch series addresses a problem where `git diff` is called using `-G` or `-S --pickaxe-regex` on new-born files that are configured without user diff drivers, and that hence get mmap()ed into memory.

The problem with that: mmap()ed memory is *not* NUL-terminated, yet the pickaxe code calls regexec() on it just the same.

This problem has been reported by my colleague Chris Sidi.

We solve this by introducing a helper, regexec_buf(), that takes a pointer and a length instead of a NUL-terminated string.

This helper then uses REG_STARTEND where available, and falls back to allocating and constructing a NUL-terminated string. Given the wide-spread support for REG_STARTEND (Linux has it, MacOSX has it, Git for Windows has it because it uses compat/regex/ that has it), I think this is a fair trade-off.

Changes since v3:

- reworded the onelines as per Junio's suggestions.

- removed fallback when REG_STARTEND is not supported, in favor of
  requiring NO_REGEX.

- removed the regmatch() function from grep.c, in favor of using
  regexec_buf().

Johannes Schindelin (3):
  regex: -G<pattern> feeds a non NUL-terminated string to regexec() and
    fails
  regex: add regexec_buf() that can work on a non NUL-terminated string
  regex: use regexec_buf()

 Makefile                |  3 ++-
 diff.c                  |  3 ++-
 diffcore-pickaxe.c      | 18 ++++++++----------
 git-compat-util.h       | 13 +++++++++++++
 grep.c                  | 14 ++------------
 t/t4061-diff-pickaxe.sh | 22 ++++++++++++++++++++++
 xdiff-interface.c       | 13 ++++---------
 7 files changed, 53 insertions(+), 33 deletions(-)
 create mode 100755 t/t4061-diff-pickaxe.sh

Published-As: https://github.com/dscho/git/releases/tag/mmap-regexec-v4
Fetch-It-Via: git fetch https://github.com/dscho/git mmap-regexec-v4

Interdiff vs v3:

 diff --git a/Makefile b/Makefile
 index df4f86b..c6f7f66 100644
 --- a/Makefile
 +++ b/Makefile
 @@ -301,7 +301,8 @@ all::
  # crashes due to allocation and free working on different 'heaps'.
  # It's defined automatically if USE_NED_ALLOCATOR is set.
  #
 -# Define NO_REGEX if you have no or inferior regex support in your C library.
 +# Define NO_REGEX if your C library lacks regex support with REG_STARTEND
 +# feature.
  #
  # Define HAVE_DEV_TTY if your system can open /dev/tty to interact with the
  # user.
 diff --git a/git-compat-util.h b/git-compat-util.h
 index 627ec5f..8aab0c3 100644
 --- a/git-compat-util.h
 +++ b/git-compat-util.h
 @@ -977,25 +977,17 @@ void git_qsort(void *base, size_t nmemb, size_t size,
  #define qsort git_qsort
  #endif
  
 +#ifndef REG_STARTEND
 +#error "Git requires REG_STARTEND support. Compile with NO_REGEX=NeedsStartEnd"
 +#endif
 +
  static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
  			      size_t nmatch, regmatch_t pmatch[], int eflags)
  {
 -#ifdef REG_STARTEND
  	assert(nmatch > 0 && pmatch);
  	pmatch[0].rm_so = 0;
  	pmatch[0].rm_eo = size;
  	return regexec(preg, buf, nmatch, pmatch, eflags | REG_STARTEND);
 -#else
 -	char *buf2 = xmalloc(size + 1);
 -	int ret;
 -
 -	memcpy(buf2, buf, size);
 -	buf2[size] = '\0';
 -	ret = regexec(preg, buf2, nmatch, pmatch, eflags);
 -	free(buf2);
 -
 -	return ret;
 -#endif
  }
  
  #ifndef DIR_HAS_BSD_GROUP_SEMANTICS
 diff --git a/grep.c b/grep.c
 index d7d00b8..1194d35 100644
 --- a/grep.c
 +++ b/grep.c
 @@ -898,17 +898,6 @@ static int fixmatch(struct grep_pat *p, char *line, char *eol,
  	}
  }
  
 -static int regmatch(const regex_t *preg, char *line, char *eol,
 -		    regmatch_t *match, int eflags)
 -{
 -#ifdef REG_STARTEND
 -	match->rm_so = 0;
 -	match->rm_eo = eol - line;
 -	eflags |= REG_STARTEND;
 -#endif
 -	return regexec(preg, line, 1, match, eflags);
 -}
 -
  static int patmatch(struct grep_pat *p, char *line, char *eol,
  		    regmatch_t *match, int eflags)
  {
 @@ -919,7 +908,8 @@ static int patmatch(struct grep_pat *p, char *line, char *eol,
  	else if (p->pcre_regexp)
  		hit = !pcrematch(p, line, eol, match, eflags);
  	else
 -		hit = !regmatch(&p->regexp, line, eol, match, eflags);
 +		hit = !regexec_buf(&p->regexp, line, eol - line, 1, match,
 +				   eflags);
  
  	return hit;
  }