Memory corruption due to word sharing

[Jan Kara]

  Hello,

  we've spotted the following mismatch between what kernel folks expect
from a compiler and what GCC really does, resulting in memory corruption on
some architectures. Consider the following structure:
struct x {
    long a;
    unsigned int b1;
    unsigned int b2:1;
};

We have two processes P1 and P2 where P1 updates field b1 and P2 updates
bitfield b2. The code GCC generates for b2 = 1 e.g. on ia64 is:
   0:   09 00 21 40 00 21       [MMI]       adds r32=8,r32
   6:   00 00 00 02 00 e0                   nop.m 0x0
   c:   11 00 00 90                         mov r15=1;;
  10:   0b 70 00 40 18 10       [MMI]       ld8 r14=[r32];;
  16:   00 00 00 02 00 c0                   nop.m 0x0
  1c:   f1 70 c0 47                         dep r14=r15,r14,32,1;;
  20:   11 00 38 40 98 11       [MIB]       st8 [r32]=r14
  26:   00 00 00 02 00 80                   nop.i 0x0
  2c:   08 00 84 00                         br.ret.sptk.many b0;;

Note that gcc used 64-bit read-modify-write cycle to update b2. Thus if P1
races with P2, update of b1 can get lost. BTW: I've just checked on x86_64
and there GCC uses 8-bit bitop to modify the bitfield.

We actually spotted this race in practice in btrfs on structure
fs/btrfs/ctree.h:struct btrfs_block_rsv where spinlock content got
corrupted due to update of following bitfield and there seem to be other
places in kernel where this could happen.

I've raised the issue with our GCC guys and they said to me that: "C does
not provide such guarantee, nor can you reliably lock different
structure fields with different locks if they share naturally aligned
word-size memory regions.  The C++11 memory model would guarantee this,
but that's not implemented nor do you build the kernel with a C++11
compiler."

So it seems what C/GCC promises does not quite match with what kernel
expects. I'm not really an expert in this area so I wanted to report it
here so that more knowledgeable people can decide how to solve the issue...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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