__copy_tofrom_user fails on unaligned read faults
paulus at samba.org
Sat Nov 23 12:03:36 EST 2002
Dale Farnsworth writes:
> copy_from_user is supposed to transfer as much data as is
> valid and then to return the number of bytes not tranferred.
> That's how it works on x86. On ppc it can be as much as 15
> bytes short.
> I initially saw the problem with the mount system call. Here's
> a partial strace:
I've always disliked the way that the mount system call does that.
However, we just have to deal with it, I guess.
IMO you are right in thinking that we need to try to copy bytes one at
a time after we get a read fault. I think I would do the extra byte
copy loop inline after the 99: label instead of jumping back the way
you do. Could you try this patch, please?
diff -urN linuxppc_2_4_devel/arch/ppc/lib/string.S pmac/arch/ppc/lib/string.S
--- linuxppc_2_4_devel/arch/ppc/lib/string.S 2002-08-13 21:52:53.000000000 +1000
+++ pmac/arch/ppc/lib/string.S 2002-11-23 12:01:31.000000000 +1100
@@ -567,10 +567,19 @@
99: mfctr r0
- add r3,r3,r5
+ add. r3,r3,r5
+ beq 120f /* shouldn't happen */
-/* for read fault, clear out the destination: r3 bytes starting at 4(r6) */
+/* for a read fault, first try to continue the copy one byte at a time */
+ mtctr r3
+130: lbz r0,4(r4)
+131: stb r0,4(r6)
+ addi r4,r4,1
+ addi r6,r6,1
+ bdnz 130b
+/* then clear out the destination: r3 bytes starting at 4(r6) */
+132: mfctr r3
@@ -591,6 +600,8 @@
+ .long 130b,132b
+ .long 131b,120b
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