From: Mark Wielaard Date: Thu, 3 Sep 2015 08:50:58 +0000 (+0200) Subject: readelf: handle_core_item large right shift triggers undefined behaviour. X-Git-Tag: elfutils-0.164~44 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b00a4fa78779ff0f304fa6cb34d49622679c86d4;p=thirdparty%2Felfutils.git readelf: handle_core_item large right shift triggers undefined behaviour. The problem is this: int n = ffs (w); w >>= n; The intent is to shift away up to (and including) the first least significant bit in w. But w is an unsigned int, so 32 bits. And the least significant bit could be bit 32 (ffs counts from 1). Unfortunately a right shift equal to (or larger than) the length in bits of the left hand operand is undefined behaviour. We expect w to be zero afterwards. Which would terminate the while loop in the function. But since it is undefined behaviour anything can happen. In this case, what will actually happen is that w is unchanged, causing an infinite loop... gcc -fsanitize=undefined will catch and warn about this when w = 0x80000000 https://bugzilla.redhat.com/show_bug.cgi?id=1259259 Signed-off-by: Mark Wielaard --- diff --git a/src/ChangeLog b/src/ChangeLog index 5be107500..66f7ead1a 100644 --- a/src/ChangeLog +++ b/src/ChangeLog @@ -1,3 +1,7 @@ +2015-09-03 Mark Wielaard + + * readelf.c (handle_core_item): Handle right shift >= 32 bits. + 2015-08-11 Mark Wielaard * elflint.c (check_sections): When gnuld and a NOBITS section falls diff --git a/src/readelf.c b/src/readelf.c index d3c2b6b4a..aab8b5ccd 100644 --- a/src/readelf.c +++ b/src/readelf.c @@ -8474,8 +8474,16 @@ handle_core_item (Elf *core, const Ebl_Core_Item *item, const void *desc, unsigned int w = negate ? ~*i : *i; while (w != 0) { - int n = ffs (w); - w >>= n; + /* Note that a right shift equal to (or greater than) + the number of bits of w is undefined behaviour. In + particular when the least significant bit is bit 32 + (w = 0x8000000) then w >>= n is undefined. So + explicitly handle that case separately. */ + unsigned int n = ffs (w); + if (n < sizeof (w) * 8) + w >>= n; + else + w = 0; bit += n; if (lastbit != 0 && lastbit + 1 == bit)