Re: [PATCH v6 2/2] Extend struct r_debug to support multiple namespaces

[Florian Weimer <fweimer@redhat.com>]

* H. J. Lu:

> On Mon, Sep 6, 2021 at 2:39 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * H. J. Lu:
>>
>> > +* The r_version update in the debugger interface makes the glibc binary
>> > +  incompatible with GDB binaries built without the following commits:
>> > +
>> > +  c0154a4a21a gdb: Don't assume r_ldsomap when r_version > 1 on Linux
>> > +  4eb629d50d4 gdbserver: Check r_version < 1 for Linux debugger interface
>>
>> Does this incompatibility happen even if audit modules and dlmopen are
>> not used?
>
> Yes.

Why?  Can't we keep r_version at 1 in this case?

>> This puts the assember output from the compiler through the
>> preprocessor.  That seems to be brittle.  I think you would have to
>> preprocess the manually written fragment separately.
>>
>> However, I think we are overdesigning things here.  The following in
>> dl-debug-symbols-gen.c should work (and the file should have a different
>> name then):
>>
>> /* Alias _r_debug to a prefix of _r_debug_extended.  */
>> asm (".set _r_debug, _r_debug_extended\n\t"
>>      ".type _r_debug, %object\n\t"
>>      ".symver _r_debug_extended, _r_debug@@" FIRST_VERSION_ld__r_debug_STRING);
>> #if __WORDSIZE == 64
>> _Static_assert (sizeof (struct r_debug) == 40, "sizeof (struct r_debug)");
>> asm (".size _r_debug, 40");
>> #else
>> _Static_assert (sizeof (struct r_debug) == 20, "sizeof (struct r_debug)");
>> asm (".size _r_debug, 20");
>> #endif
>>
>> It's not exactly pretty, but at least it's obvious what is going on.
>> (Extended asm with input operands is not supported outside of functions.)
>
> This was the first thing I tried and it didn't work:
>
> [hjl@gnu-cfl-2 tmp]$ cat foo.s
> .set  _r_debug, _r_debug_extended
> .globl _r_debug
> .type _r_debug, %object
> .size _r_debug, 40
> .data
> .type _r_debug_extended, %object
> .size _r_debug_extended, 48
> .globl _r_debug_extended
> _r_debug_extended:
> .zero 48
> [hjl@gnu-cfl-2 tmp]$ gcc -c foo.s
> [hjl@gnu-cfl-2 tmp]$ readelf -sW foo.o | grep _r_debug
>      1: 0000000000000000    48 OBJECT  GLOBAL DEFAULT    2 _r_debug
>      2: 0000000000000000    48 OBJECT  GLOBAL DEFAULT    2 _r_debug_extended
> [hjl@gnu-cfl-2 tmp]$

Huh.  Does this mean this depends on the symbol definition order in the
assembler file?

I really hate the post-processing of compiler output.  This isn't GHC. 8->

Can we write a portable assembler file instead?

Nick Clifton has written down some guidelines:

  Tips for writing portable assembler with GNU Assembler (GAS)
  <https://developers.redhat.com/blog/2021/02/26/tips-for-writing-portable-assembler-with-gnu-assembler-gas>

There's no initializer, so all we need to know is size and alignment.

>> Is this necessary?  It makes concurrent access to the list harder and
>
> When _dl_close_worker is called, it holds GL(dl_load_lock).  Why does
> this change make concurrent access harder?

Something else might want to read the list directly, by starting with
DT_DEBUG.

Thanks,
Florian