[llvm-dev] Resolve line numbers with LLVM symbolizer

[David Jones via llvm-dev]

This could get a little bit tricky due to Address Space Layout
Randomization (ASLR). llvm-symbolizer is able to map an address to
file:line:offset, but only if the address given to it matches what is in
the ELF file to begin with.

ASLR introduces randomness into the upper address bits, which makes this
impossible unless you can filter it out.

If you are using the Linux dladdr function in the process of getting a
backtrace, then it provides the base address of whatever segment the
queried function is in. (This is true at least for shared libraries; I'm
not sure if it is also true for those parts of the program that are not in
a shared library.) Subtracting the base address of the library from the
address in question will give you the offset into the library, which is the
address that llvm-symbolizer really wants.

    if (dladdr(addr, &info)) {
        // addr - info.dli_fbase is address that must be passed to
llvm-symbolizer
        string          so_name = info.dli_fname;
        intptr_t        offset = (char*)addr - (char*)info.dli_fbase;

        // invoke llvm-symbolizer with so_name and offset


On Mon, May 9, 2016 at 9:34 AM, Nathan Schagen via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi LLVM people,
>
> I would like the llvm-symbolizer to resolve both function names and
> line/column numbers for me. The latter does not seem to work.
>
>  I'm using the Memory Sanitizer example with LLVM 3.3. This is what I did:
>
> ➜  msan_test cat test.c
> #include <stdio.h>
>
> int main(int argc, char** argv) {
>   int a[10];
>   a[5] = 0;
>   if (a[argc])
>     printf("xx\n");
>   return 0;
> }
> ➜  msan_test clang -fsanitize=memory -fno-omit-frame-pointer -g -O2 -o
> test test.c
> ➜  msan_test ./test
> ==9547== WARNING: Use of uninitialized value
>     #0 0x55ea59c9c476 (/home/nathan/test/c/msan_test/test+0x25476)
>     #1 0x7fbdfc187a3f (/lib/x86_64-linux-gnu/libc.so.6+0x20a3f)
>     #2 0x55ea59c9c2a8 (/home/nathan/test/c/msan_test/test+0x252a8)
> Exiting
>
> Copied file/offset pairs into report.txt. I am aware of
> MSAN_SYMBOLIZER_PATH.
>
> ➜  msan_test cat report.txt
> /home/nathan/test/c/msan_test/test 0x25476
> /lib/x86_64-linux-gnu/libc.so.6 0x20a3f
> /home/nathan/test/c/msan_test/test 0x252a8
> ➜  msan_test llvm-symbolizer < report.txt
> main
> ??:0:0
>
> ??
> ??:0:0
>
> _start
> ??:0:0
>
> ➜  msan_test
>
> It's unclear to me why no line numbers show up. This is what readelf has
> to say:
>
> ➜  msan_test readelf -WS test | egrep '\.(stab|debug)'
>   [28] .debug_info       PROGBITS        0000000000000000 02ec10 040d9e 00
>      0   0  1
>   [29] .debug_abbrev     PROGBITS        0000000000000000 06f9ae 006943 00
>      0   0  1
>   [30] .debug_loc        PROGBITS        0000000000000000 0762f1 0637d3 00
>      0   0  1
>   [31] .debug_aranges    PROGBITS        0000000000000000 0d9ac4 000570 00
>      0   0  1
>   [32] .debug_ranges     PROGBITS        0000000000000000 0da034 014fe0 00
>      0   0  1
>   [33] .debug_line       PROGBITS        0000000000000000 0ef014 0091de 00
>      0   0  1
>   [34] .debug_str        PROGBITS        0000000000000000 0f81f2 010e90 01
>  MS  0   0  1
>   [36] .debug_macinfo    PROGBITS        0000000000000000 1090cd 000000 00
>      0   0  1
>   [37] .debug_pubtypes   PROGBITS        0000000000000000 1090cd 000000 00
>      0   0  1
>
> Can someone point me in the right direction?
>
> Thanks in advance!
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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