On 13.08.2012 21:02, Rainer Jung wrote:
On 13.08.2012 19:40, Jeff Trawick wrote:
On Mon, Aug 13, 2012 at 1:27 PM, Rainer Jung <rainer.j...@kippdata.de>
wrote:
On 13.08.2012 18:32, Jeff Trawick wrote:
On Mon, Aug 13, 2012 at 12:30 PM, Rainer Jung <rainer.j...@kippdata.de>
wrote:
Hi,
PR 53040 reveals, that mod_socache_shmcb has an alignment problem.
One of
the three structs mapped into shm contains an apr_time_t member,
which at
least on Sparc is 8 Bytes, whereas for 32 bit builds long is only 4
Bytes.
Currently everything is aligned for 4 Bytes, so we get bus
errors/crashes
when trying to assign the apr_time_t to an address that is only
divisible
by
4 instead of 8.
I can easily reproduce the problem.
A possible solution is to pad the three structures SHMCBHeader,
SHMCBSubcache and SHMCBIndex to a multiple of 8 Bytes length. For
Subcache
and Index this is already true by coincidence, SHMCBHeader needs
another
4
Bytes.
I wonder what the right solution is. In the patch
http://people.apache.org/~rjung/patches/mod_socache_shmcb-padding.patch
I hard coded the padding, but I don't really like it, because it
breaks
if
members are added to the struct. I could add a sizeof() test during
startup
or probably even compilation to warn or err, if the padding is wrong.
I see several recipes for alignment using pragmas and attribute,
but all
of
them are compiler specific.
One could also wrap the struct in a wrapped struct, so that one
could use
the sizeof() of the inner struct to determine the padding of the outer
struct. That would make the code convoluted.
I checked other parts of the code, but couldn't find a simple
solution.
Any
hints how to do this nicely?
APR_ALIGN_DEFAULT?
I think it doesn't solve this problem, does it? It only gives me a
need way
to round up sizes to multiples of 8 bytes.
It should be possible to use use APR_ALIGN_DEFAULT(sizeof(foo)) in
place of sizeof(foo), instead of adding explicit padding to the
structures. Any other pointer arithmetic which doesn't use sizeof(foo)
would also need to use the macro.
Understood. Unfortunately currently the code doesn't really care about
alignment. So it just puts structs SHMCBHeader, SHMCBSubcache and an
array of SHMCBIndex directly after each other in shm. No sizeof() involved.
So yes, option 3 would be to rewrite the code to calculate the
gaps/padding between the structs using APR_ALIGN_DEFAULT() and adjust
memory alignment in shm. I wanted to get around changing that part of
the code ;)
I went the "choose right alignment" way now:
http://people.apache.org/~rjung/patches/mod_socache_shmcb-alignment.patch
It actually wasn't that complicated.
Regards,
Rainer
