Re: [PATCH] readelf: handle_core_item large right shift triggers undefined behaviour.

Re: [PATCH] readelf: handle_core_item large right shift triggers undefined behaviour.

[Mark Wielaard]

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On Fri, 2015-09-04 at 09:33 +0200, Mark Wielaard wrote:
> I prefer the simpler: if (n < sizeof (w) * 8) w >>= n; else w = 0;
> Which simply states what value we expect for w.

I pushed this version to master now.

Re: [PATCH] readelf: handle_core_item large right shift triggers undefined behaviour.

[Mark Wielaard]

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On Thu, 2015-09-03 at 13:58 -0700, Roland McGrath wrote:
> What about:
> 	w >>= n % 32;
> 	w >>= (n - (n % 32)) % 32;
> ?

I don't think that works. For the interesting case n == 32, you won't
shift at all. Because both expressions (32 % 32) and ((32 - 0) % 32)
evaluate to zero.
I prefer the simpler: if (n < sizeof (w) * 8) w >>= n; else w = 0;
Which simply states what value we expect for w.

Cheers,

Mark

Re: [PATCH] readelf: handle_core_item large right shift triggers undefined behaviour.

[Roland McGrath]

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What about:
	w >>= n % 32;
	w >>= (n - (n % 32)) % 32;
?

[PATCH] readelf: handle_core_item large right shift triggers undefined behaviour.

[Mark Wielaard]

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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 <mjw@redhat.com>
---
 src/ChangeLog |  4 ++++
 src/readelf.c | 12 ++++++++++--
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/ChangeLog b/src/ChangeLog
index 5be1075..66f7ead 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,3 +1,7 @@
+2015-09-03  Mark Wielaard  <mjw@redhat.com>
+
+	* readelf.c (handle_core_item): Handle right shift >= 32 bits.
+
 2015-08-11  Mark Wielaard  <mjw@redhat.com>
 
 	* elflint.c (check_sections): When gnuld and a NOBITS section falls
diff --git a/src/readelf.c b/src/readelf.c
index d3c2b6b..aab8b5c 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)
-- 
2.4.3