On Thu, Mar 29, 2018 at 10:28:24AM +0100, Matt Redfearn wrote:
The __clear_user function is defined to return the number of bytes that could not be cleared. From the underlying memset / bzero implementation this means setting register a2 to that number on return. Currently if a page fault is triggered within the memset_partial block, the value loaded into a2 on return is meaningless.
The label .Lpartial_fixup@ is jumped to on page fault. Currently it masks the remaining count of bytes (a2) with STORMASK, meaning that the least significant 2 (32bit) or 3 (64bit) bits of the remaining count are always clear.
Are you sure about that. It seems to do that *to ensure those bits are set correctly*...
Secondly, .Lpartial_fixup@ expects t1 to contain the end address of the copy. This is set up by the initial block: PTR_ADDU t1, a0 /* end address */ However, the .Lmemset_partial@ block then reuses register t1 to calculate a jump through a block of word copies. This leaves it no longer containing the end address of the copy operation if a page fault occurs, and the remaining bytes calculation is incorrect.
Fix these issues by removing the and of a2 with STORMASK, and replace t1 with register t2 in the .Lmemset_partial@ block.
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2") Cc: stable@vger.kernel.org Signed-off-by: Matt Redfearn matt.redfearn@mips.com
arch/mips/lib/memset.S | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-)
diff --git a/arch/mips/lib/memset.S b/arch/mips/lib/memset.S index 90bcdf1224ee..3257dca58cad 100644 --- a/arch/mips/lib/memset.S +++ b/arch/mips/lib/memset.S @@ -161,19 +161,19 @@ .Lmemset_partial@: R10KCBARRIER(0(ra))
- PTR_LA t1, 2f /* where to start */
- PTR_LA t2, 2f /* where to start */
#ifdef CONFIG_CPU_MICROMIPS LONG_SRL t7, t0, 1
Hmm, on microMIPS t7 isn't on the clobber list for __bzero, and nor is t8...
#endif #if LONGSIZE == 4
- PTR_SUBU t1, FILLPTRG
- PTR_SUBU t2, FILLPTRG
#else .set noat LONG_SRL AT, FILLPTRG, 1
- PTR_SUBU t1, AT
- PTR_SUBU t2, AT .set at
#endif
- jr t1
- jr t2 PTR_ADDU a0, t0 /* dest ptr */
^^^ note this...
.set push @@ -250,7 +250,6 @@ .Lpartial_fixup@: PTR_L t0, TI_TASK($28)
- andi a2, STORMASK
... this isn't right.
If I read correctly, t1 (after the above change stops clobbering it) is the end of the full 64-byte blocks, i.e. the start address of the final partial block.
The .Lfwd_fixup calculation (for full blocks) appears to be:
a2 = ((len & 0x3f) + start_of_partial) - badvaddr
which is spot on. (len & 0x3f) is the partial block and remaining bytes that haven't been set yet, add start_of_partial to get end of the full range, subtract bad address to find how much didn't copy.
The calculation for .Lpartial_fixup however appears to (currently) do:
a2 = ((len & STORMASK) + start_of_partial) - badvaddr
Which might make sense if start_of_partial (t1) was replaced with end_of_partial, which does seem to be calculated as noted above, and put in a0 ready for the final few bytes to be set.
LONG_L t0, THREAD_BUADDR(t0) LONG_ADDU a2, t1
^^ So I think either it needs to just s/t1/a0/ here and not bother preserving t1 above (smaller change and probably the original intent), or preserve t1 and mask 0x3f instead of STORMASK like .Lfwd_fixup does (which would work but seems needlessly complicated to me).
Does that make any sense or have I misunderstood some subtlety?
Cheers James
jr ra
2.7.4