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Subjectgcc bug causing problem in kernel builds
We have been experiencing some problems trying to use kernel modules
with kernels that are compiled with different versions of gcc. On our
kernel build machine (where we compile our kernel modules) we have gcc
2.91.66 (I believe the preferred kernel compiler, according to
Documentation/Changes); RedHat 7.1 ships with gcc 2.96.

Now, the problem is that RedHat also apparently compiles (at least its
newer) kernels with the 2.96 gcc. Unfortunately, there appears to be a
structure misalignment problem in gcc 2.96.

One particular instance of this problem that we are running into is in
the raid1.o module in the 2.4.3 kernel. The structure alignment problem
is causing our gcc 2.91.66-compiled raid1 module to malfunction.
(raid1.o compiled from the same source on gcc 2.96 works fine.) We've
traced the problem down to the following assembly code generated by the
2.96 and 2.91.66 gcc's respectively:

(assembly code for parameter setup and call to __alloc_pages (within
raid1_grow_buffers))

2.96:

movl $contig_page_data_Rsmp_cef82582+3800, %eax
call __alloc_pages_Rsmp_decacc2f

2.91.66:

movl $contig_page_data_Rsmp_cef82582+3884,%eax
call __alloc_pages_Rsmp_decacc2f


gcc 2.91.66 is padding out the zone_t structure by 28 bytes. With an
array of 3 of those before our field in question that equals 84 bytes
offset in the above assembler code.

The 28 byte padding is because gcc 2.91.66 is trying to 32 byte align
this structure. The reason for this is that the first submember of
zone_t is explicitly defined as 32 byte aligned (per_cpu_t).

So, gcc 2.91.66 is (properly) aligning the per_cpu_t structure on a 32
byte boundary as specified by the __attribute__((aligned(32))) directive
in that structure's definition:

(gdb) p &((pg_data_t *)0)->node_zones[1].cpu_pages[0]
$22 = (per_cpu_t *) 0x4e0

(gdb) p &((pg_data_t *)0)->node_zones[1].cpu_pages[1]
$23 = (per_cpu_t *) 0x500

(gdb) p 0x500 % 32
$24 = 0

(gdb) p 0x4e0 % 32
$25 = 0


gcc 2.96 is not properly aligning this structure:

(gdb) p &((pg_data_t *)0)->node_zones[1].cpu_pages[0]
$32 = (per_cpu_t *) 0x4c4

(gdb) p &((pg_data_t *)0)->node_zones[1].cpu_pages[1]
$33 = (per_cpu_t *) 0x4e4

(gdb) p 0x4c4 % 32
$34 = 4

(gdb) p 0x4e4 % 32
$35 = 4



So, in order for our raid1 modules to work properly with a kernel
compiled by gcc 2.96, we must also use (the broken) 2.96 to compile our
module.


So I guess some of the questions that arise from all this are:

Does anyone know if the structure misalignment problem in gcc 2.96 is a
known issue? (could this bug be induced by a RedHat-applied gcc patch,
if there are any)

How widespread is this problem? i.e., do other distributions have
similar issues?, do other versions of gcc have similar issues?, are
there other places in the kernel where this type of problem might occur?


I'll also post to gcc bug list.

----
Paul

Paul.Clements@SteelEye.com
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