public inbox for
 help / color / mirror / Atom feed
From: Simon Marchi <>
To: Yichao Yu <>,
Subject: Re: Suspected bug in DW_OP_addr handling
Date: Sun, 8 May 2022 13:25:39 -0400	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On 2022-05-06 23:54, Yichao Yu via Gdb wrote:
> I noticed that gdb unwind failed with read of invalid memory address
> when I used `DW_OP_addr` in my unwind info. Upon checking, it seems
> that the handling of this operation in the dwarf interpreter is very
> suspicious.
> The interpreter for the op code has a comment[1] sayijng,
>> Some versions of GCC emit DW_OP_addr before
>> DW_OP_GNU_push_tls_address. In this case the value is an
>> index, not an address.
> However, the code appears to check for exactly the opposite condition
> `op_ptr >= op_end || *op_ptr != DW_OP_GNU_push_tls_address` to decide
> whether the result should be used as index. This was added 12 years
> ago in commit ac56253ddece [2].
> Am I missing something or is this a long-standing bug?

My understanding is:

 - normally, a value given by DW_OP_addr is an address that should be
   relocated with the base address of the objfile
 - the DW_OP_GNU_push_tls_address operation (now called
   DW_OP_form_tls_address in DWARF 5) expects some kind of value on top
   of the stack, identifying the TLS variable to fetch.  It's
   implementation-defined what this value means, but in practice it
   means it's some value that should not be relocated.
 - a contemporary version of GCC produces something like this for a TLS
   variable's DW_AT_location:

     DW_OP_const8u 0x4, DW_OP_form_tls_address

 - A version of gcc in the past must have used this instead:

     DW_OP_addr 0x4, DW_OP_form_tls_address

The usage of DW_OP_addr was wrong, and it made so GDB had to avoid
relocating the DW_OP_addr value in this particular case.  So it is
checking, if we have DW_OP_addr followed by DW_OP_form_tls_address, then
we don't relocate, because we are in this buggy situation.

And so the condition:

	  if (op_ptr >= op_end || *op_ptr != DW_OP_GNU_push_tls_address)
	    result += this->m_per_objfile->objfile->text_section_offset ();

looks right to me.  It says, only relocate if:

 - DW_OP_addr is the last operation of the sequence
 - the following op is not DW_OP_form_tls_address / DW_OP_GNU_push_tls_address


  reply	other threads:[~2022-05-08 17:25 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-07  3:54 Yichao Yu
2022-05-08 17:25 ` Simon Marchi [this message]
2022-05-09 14:03   ` Yichao Yu

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).