[Mesa-dev] [PATCH] glcpp: Don't use alternation in the lookahead for empty pragmas.

Carl Worth cworth at cworth.org
Mon Aug 18 11:49:13 PDT 2014

We've found that there's a buffer overrun bug in flex that's triggered by
using alternation in a lookahead pattern.

Fortunately, we don't need to match the exact {NEWLINE} expression to detect
an empty pragma. It suffices to verify that there are no non-space characters
before any newline character. So we can use a simple [\r\n] to get the desired
behavior while avoiding the flex bug.

Fixes Piglit's 16385-consecutive-chars and
17000-consecutive-chars-identifier tests.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82472
Signed-off-by: Carl Worth <cworth at cworth.org>
Approach-suggested-by: Kenneth Graunke <kenneth at whitecape.org>
CC: Kenneth Graunke <kenneth at whitecape.org>

 Thanks for chasing down the fix for this regression of mine, Ken.

 I am embarrassed that I clearly didn't run piglit enough while testing my
 original branch.

 With your fix above, there is some state that's not updated as it should
 be when returning a NEWLINE token, (such as incrementing yylineno, etc.).
 I tried to improve things to update all that state, but it proved
 problematic, (putting the state updates in a common function doesn't
 work because only the outer lexing function has access to local variables
 like yylineno).

 The alternate approach here was your recommendation, of course.

 src/glsl/glcpp/glcpp-lex.l | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/src/glsl/glcpp/glcpp-lex.l b/src/glsl/glcpp/glcpp-lex.l
index 98d500e..aaef7b8 100644
--- a/src/glsl/glcpp/glcpp-lex.l
+++ b/src/glsl/glcpp/glcpp-lex.l
@@ -289,8 +289,14 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 	/* Swallow empty #pragma directives, (to avoid confusing the
-	 * downstream compiler). */
-<HASH>pragma{HSPACE}*/{NEWLINE} {
+	 * downstream compiler).
+	 *
+	 * Note: We use a simple regular expression for the lookahead
+	 * here. Specifically, we cannot use the complete {NEWLINE} expression
+	 * since it uses alternation and we've found that there's a flex bug
+	 * where using alternation in the lookahead portion of a pattern
+	 * triggers a buffer overrun.  /
+<HASH>pragma{HSPACE}*/[\r\n] {

More information about the mesa-dev mailing list