BUS error on misaligned memory access on some ARM

[Mike Z]

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

[Carl Eugen Hoyos]

Thank you for porting this from ffmpeg-user!

Could you add complete output, ie everything (including command line, maybe r -i file -f null -) above "Program received"?

[Mike Z]

There is no command line, it's part of an android application. ​http://code.google.com/p/jjmpeg/

This is just a bug in the configure script in that it assumes that armv6+ supports fast unaligned loads - whereas armv6+ linux kernels can be configured to bus error on unaligned loads.

[Michael Niedermayer]

added a --disable-fast-unaligned option

[jacob hameiri]

notzed, does 'disable-fast-unaligned' actually resolve the bus error ? I am using latest ffmpeg (1.0) in a android arm cpu tablet with disable-fast-unaligned and still getting bus error.

[Carl Eugen Hoyos]

Replying to jacob:

I am using latest ffmpeg (1.0) in a android arm cpu tablet with disable-fast-unaligned and still getting bus error.

Please provide command line together with complete, uncut console output and backtrace etc. as explained on https://ffmpeg.org/bugreports.html

[Mike Z]

I don't know, i only tested it on 0.11.x, and my build still sets the value manually.

Since it worked for me, i presume michael's change did the same and should work too.

[jacob hameiri]

Replying to notzed:

I don't know, i only tested it on 0.11.x, and my build still sets the value manually.

Since it worked for me, i presume michael's change did the same and should work too.

notzed, can you please send me your build, I will check with it.

I know I should past the full log here but it is very long then I will use pastebin.

​http://pastebin.com/1H6H6Ync

after that ffmpeg prints "Bus error essage repeated 4 times " and exists.

[Carl Eugen Hoyos]

Replying to jacob:

I know I should past the full log here but it is very long then I will use pastebin.

​http://pastebin.com/1H6H6Ync

Output on pastebin will disappear one day, if somebody (as you have been yesterday) is interested in this ticket in the future, how will he understand your problem?
Consider attaching the output as a text file if it is too long.

I don't see anything wrong with the output on pastebin, it looks incomplete though. If you encounter a crash, please add the missing information, see ​http://ffmpeg.org/bugreports.html

Is network output necessary to reproduce the problem, or is file output sufficient?

[Mike Z]

I'm not using ffmpeg, i'm using libavcodec and friends. The project source is linked in an earlier comment, it includes the build file.