[PATCH] Fix libbacktrace on 32-bit sparc

[David Miller]

From: David Miller <davem@davemloft.net>
Date: Sun, 28 Oct 2012 00:31:27 -0400 (EDT)

> The size is 24, and my patch definitely makes the crashes go away.
> 
> It seems like a vector is being used for a mixed set of objects.
> I'll try to figure out how that is happening.

Ok, the problem seems to have to do with releases.

The releases place vector memory chunks into a global pool.

So a memory chunk from a vector used for one type of object,
can be sucked into and used by another vector.

But the alignment requirements are different, so we can
obtain a chunk from the freelist that was being used for
a vector of 4-byte aligned objects.

The crash sequences are always of the form:

vec_release(0xffb37ac8) base+size(0xf0199008) amount(312)
...
vec_grow(0xffb37ac8:24) from 0x975168, ret=0xf01754cc [size(24):alc(360)]

That size alignment done by backtrace_alloc() has no influence upon
this issue.  Since chunks are released from wherever the vector's
allocation point was at the time of the release.

In fact I bet that alignment in backtrace_alloc() never triggers when
it is invoked from backtrace_vector_grow().