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Date: Tue, 19 Jan 2016 13:45:31 +0100
From: Felix Fietkau <nbd@...nwrt.org>
To: musl@...ts.openwall.com
Cc: musl@...dco.com
Subject: Re: MUSCL + MIPS + threads + debugging/stack traces
On 2016-01-19 05:19, Rich Felker wrote:
> On Mon, Jan 18, 2016 at 05:34:27PM -0800, Ward Willats wrote:
>> Hello.
>>
>> We're working on a consumer product using an OpenWRT-based MIPS
>> Linux platform and linking our C++11 std::thread (really pthread)
>> app to MUSL.
>>
>> When we try to get a backtrace with the toolchain gdb/gdbserver (or
>> the latest one we cross compiled ourselves from source) we get these
>> kind of pathetic, truncated stack traces.
>>
>> Here, for example, is a sample:
>>
>> (gdb) thread apply all backtrace
>>
>> Thread 4 (Thread 8009.9567):
>> #0 0x77fcb904 in __cp_end () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> #1 0x77fc6f38 in __syscall_cp_c () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> #2 0x00000000 in ?? ()
>> Backtrace stopped: frame did not save the PC
>>
>> Thread 3 (Thread 8009.9566):
>> #0 0x77fcb904 in __cp_end () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> #1 0x77fc6f38 in __syscall_cp_c () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> #2 0x00000000 in ?? ()
>> Backtrace stopped: frame did not save the PC
>>
>> Thread 2 (Thread 8009.9565):
>> #0 0x77fcb904 in __cp_end () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> #1 0x77fc6f38 in __syscall_cp_c () from /Volumes/OpenWrt/fsp-openwrt/staging_dir/target-mipsel_24kec+dsp_musl-1.1.11/root-UberFSP/lib/ld-musl-mipsel-sf.so.1
>> warning: GDB can't find the start of the function at 0x77b8da46.
>>
>> GDB is unable to find the start of the function at 0x77b8da46
>> and thus can't determine the size of that function's stack frame.
>> This means that GDB may be unable to access that stack frame, or
>> the frames below it.
>> This problem is most likely caused by an invalid program counter or
>> stack pointer.
>> However, if you think GDB should simply search farther back
>> from 0x77b8da46 for code which looks like the beginning of a
>> function, you can increase the range of the search using the `set
>> heuristic-fence-post' command.
>> #2 0x77b8da48 in ?? ()
>
> It looks like your libc build lacks debug info and thus gdb is
> attempting to use heuristics to make sense of the call stack. Do you
> know if this is the case, and if so, whether there's a way to get
> OpenWRT to build libc with -g? It's also possible that missing CFI in
> our asm files it the cause. I'll ask and see if anyone else familiar
> with MIPS has run into this and has ideas.
Seems that I forgot to put in --enable-debug in the Makefile. This is
fixed in r48363 now.
- Felix
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