#1481: BUS error on misaligned memory access on some ARM
-------------------------------------+-------------------------------------
Reporter: notzed | Type: defect
Status: new | Priority: normal
Component: build | Version:
system | unspecified
Keywords: | Blocked By:
Blocking: | Reproduced by developer: 0
Analyzed by developer: 0 |
-------------------------------------+-------------------------------------
By default configure on armv6+ enables misaligned memory accesses using
the HAVE_FAST_UNALIGNED define.
I think some(?) ARM cpu's have run-time configurable exception behaviour
for misaligned memory accesses, and unaligned access causes BUS errors if
so configured. On locked android machines this cannot be changed by the
user, and in general it's a global system setting so not appropriate to
change anyway. Crash is reported for a Tegra 3 tablet - ASUS transformer
prime.
example crash below, where:
ldr r3, [r0, r1, lsr #3]
equates to: *((unsigned int *)(r0 + r1 >> 3))
Strangely this error is still quite rare and un-predictable, so I'm not
certain this is the cause. But manually disabling HAVE_FAST_UNALIGNED
seems to fix it.
I don't really know what to suggest, perhaps allow HAVE_FAST_UNALIGNED to
be overridden by a configure option.
Program received signal SIGBUS, Bus error.
[Switching to Thread 12614]
mpeg4_decode_block (s=0x1f97f40, block=<value optimized out>, n=1576,
coded=<value optimized out>, intra=488, rvlc=1534662004) at
/home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavutil/arm/intreadwrite.h:54
54 __asm__ ("ldr %0, %1" : "=r"(v) : "m"(*q));
(gdb) where
#0 mpeg4_decode_block (s=0x1f97f40, block=<value optimized out>, n=1576,
coded=<value optimized out>, intra=488, rvlc=1534662004) at
/home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavutil/arm/intreadwrite.h:54
#1 0x5b791574 in mpeg4_decode_mb (s=0x1f97f40, block=<value optimized
out>) at /home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavcodec/mpeg4videodec.c:1486
#2 0x5b710860 in decode_slice (s=0x1f97f40) at
/home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavcodec/h263dec.c:217
#3 0x5b711b4c in ff_h263_decode_frame (avctx=0x1f66570, data=<value
optimized out>, data_size=<value optimized out>, avpkt=<value optimized
out>) at /home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavcodec/h263dec.c:675
#4 0x5b7cfab8 in frame_worker_thread (arg=<value optimized out>) at
/home/notzed/svn/jjmpeg-0.11/jjmpeg-
core/jni/ffmpeg-0.11/libavcodec/pthread.c:381
#5 0x40073e30 in __thread_entry () from /home/notzed/svn/jjmpeg-0.11
/jjmpeg-android/obj/local/armeabi-v7a/libc.so
#6 0x40073984 in pthread_create () from /home/notzed/svn/jjmpeg-0.11
/jjmpeg-android/obj/local/armeabi-v7a/libc.so
#7 0x00000000 in ?? ()
(gdb) disassemble $pc-16 $pc+16
Dump of assembler code from 0x5b790514 to 0x5b790534:
0x5b790514 <mpeg4_decode_block+532>: ldr r1, [r4, r3]
0x5b790518 <mpeg4_decode_block+536>: movw r12, #10732 ; 0x29ec
0x5b79051c <mpeg4_decode_block+540>: and r2, r1, #7 ; 0x7
0x5b790520 <mpeg4_decode_block+544>: ldr r0, [r4, r12]
0x5b790524 <mpeg4_decode_block+548>: ldr r3, [r0, r1, lsr #3]
0x5b790528 <mpeg4_decode_block+552>: rev r3, r3
0x5b79052c <mpeg4_decode_block+556>: mov r3, r3, lsl r2
0x5b790530 <mpeg4_decode_block+560>: mov r12, r3, lsr #23
End of assembler dump.
(gdb) info registers all
r0 0x21d18c0 35461312
r1 0x9998 39320
r2 0x0 0
r3 0x12 18
r4 0x1f97f40 33128256
r5 0x4 4
r6 0x5bb01568 1538266472
r7 0x628 1576
r8 0x1 1
r9 0x2314 8980
r10 0x0 0
r11 0x1 1
r12 0x29ec 10732
sp 0x5ca95ce8 0x5ca95ce8
lr 0x5b791574 1534662004
pc 0x5b790524 0x5b790524 <mpeg4_decode_block+548>
f0 0 (raw 0x000000000000000000000000)
f1 0 (raw 0x000000000000000000000000)
f2 0 (raw 0x000000000000000000000000)
f3 0 (raw 0x000000000000000000000000)
f4 0 (raw 0x000000000000000000000000)
f5 0 (raw 0x000000000000000000000000)
f6 0 (raw 0x000000000000000000000000)
f7 0 (raw 0x000000000000000000000000)
fps 0x0 0
cpsr 0x80000010 2147483664
--
Ticket URL: <https://ffmpeg.org/trac/ffmpeg/ticket/1481>
FFmpeg <http://ffmpeg.org>
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