Re: [PATCH] [PR 25678] gdb crashes with "internal-error: sect_index_text not initialized" when .text

Simon Marchi
2020-05-19T14:50:13Z

On 2020-05-19 10:36 a.m., mlimber wrote:
> I have skimmed that code, but we're beyond my ken here. The things I observe in that code are:
>
> 1. This only acts on files with 1 or 2 segments. (It gets skipped for libtestcase.so as you say.)
>
> 2. If the segment info is 1 or 2, it sets two segment indices to refer to this one segment. Perhaps that's legit (I'm a naif when it comes to ELF details), but it struck me as odd.
>
> 3. Line 300, where this function is called, has this curious comment:
>
> /* This is where things get really weird... We MUST have valid
>
> indices for the various sect_index_* members or gdb will abort.
>
> So if for example, there is no ".text" section, we have to
>
> accomodate that. First, check for a file with the standard
>
> one or two segments. */

I reached the same conclusions.

>
>
>
> So I'm curious, in your libicudata.so library, how many segments there are.  That
> can be checked with:
>
>   $ readelf -l libicudata.so.52 | grep LOAD
>
>
> I have two load segments:
>
> readelf -l libicudata.so
>
> Elf file type is DYN (Shared object file)
> Entry point 0x2b6
> There are 6 program headers, starting at offset 64
>
> Program Headers:
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags  Align
>   LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
>                  0x000000000166a940 0x000000000166a940  R      200000
>   LOAD           0x000000000166af30 0x000000000186af30 0x000000000186af30
>                  0x00000000000000d0 0x00000000000000d0  RW     200000
>   DYNAMIC        0x000000000166af30 0x000000000186af30 0x000000000186af30
>                  0x00000000000000d0 0x00000000000000d0  RW     8
>   NOTE           0x0000000000000190 0x0000000000000190 0x0000000000000190
>                  0x0000000000000024 0x0000000000000024  R      4
>   GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
>                  0x0000000000000000 0x0000000000000000  RW     10
>   GNU_RELRO      0x000000000166af30 0x000000000186af30 0x000000000186af30
>                  0x00000000000000d0 0x00000000000000d0  R      1

Are we inspecting the same library? In the libicudata.so.52 you've sent, there
are three load segments:

$ readelf -l libicudata.so.52.2 | grep LOAD
LOAD 0x000000 0x0000000000000000 0x0000000000000000 0x166a940 0x166a940 R 0x200000
LOAD 0x166af30 0x000000000186af30 0x000000000186af30 0x0000d0 0x0000d0 RW 0x200000
LOAD 0x166c000 0x000000000186b000 0x000000000186b000 0x000180 0x000180 RW 0x1000

I successfully reproduced the bug using your lib. Since there's no DWARF
info, it fails in init_entry_point_info. With my lib, it fails earlier,
when the DWARF info is read. Anyway, it's all variations of the same bug,
some code assumes that sect_index_text is set to some valid value.<

> I note that there is an entry point specified on the second line of output, which is curious to me since there is no code in this library.

I noticed that too, shared libraries have entry points... that fields looks
mandatory in the ELF header, so it can probably just be ignored.

>
> Perhaps if we forced an extra load segment in this .so, it would produce different results due to skipping the function cited above.
>
>
> If the libicudata.so.52 is really the problematic one, I'm a bit surprised that you
> don't always see the problem when debugging a program that uses it.
>
>
> I'm also not sure why it sometimes happens and sometimes doesn't. Could it be something with how or when it is loaded -- say, in a certain sequence or via a manual dlopen() instead of via dynamic linking info?

The only reason I would see is that you might not be loading the libicudata.so
you think you are loading.

Simon