[PING][RFC] [gdb] Work around zero l_addr/l_ld

[Tom de Vries <tdevries@suse.de>]

On 8/18/26 9:14 AM, Tom de Vries wrote:
> On ppc64le-linux (AlmaLinux 9.8), I run into:
> ...
> FAIL: gdb.mi/mi-dlmopen.exp: still-in-use fields were all correct
> ...
> 
> While investigating this, I stumbled on this warning emitted during the
> calculation of allow_dlmopen_tests:
> ...
> (gdb) run ^M
> Starting program: allow_dlmopen_tests.x ^M
> [Thread debugging using libthread_db enabled]^M
> Using host libthread_db library "/lib64/libthread_db.so.1".^M
> warning: .dynamic section for "/lib64/ld64.so.2" is not at the expected \
>    address (wrong library or version mismatch?)^M
> dlmopen debug supported.^M
> ...
> 
> The warning is mentioned in this glibc commit 88361b408b:
> ...
>      elf: Copy l_addr/l_ld when adding ld.so to a new namespace
> 
>      When add ld.so to a new namespace, we don't actually load ld.so.  We
>      create a new link map and refers the real one for almost everything.
>      Copy l_addr and l_ld from the real ld.so link map to avoid GDB warning:
> 
>      warning: .dynamic section for ".../elf/ld-linux-x86-64.so.2" is not at \
>        the expected address (wrong library or version mismatch?)
> 
>      when handling shared library loaded by dlmopen.
> ...
> 
> So, AFAICT the setup is:
> - the glibc package is based on v2.34
> - it contains a backport of commit a93d9e03a3 ("Extend struct r_debug to
>    support multiple namespaces [BZ #15971]")
> - it doesn't contain a backport of commit 88361b408b ("elf: Copy l_addr/l_ld
>    when adding ld.so to a new namespace")
> - both commits are part of v2.35
> 
> What happens is:
> - when probing for l_addr and l_ld in svr4_solib_ops::read_lm_info, both get
>    the value 0
> - in svr4_solib_ops::lm_addr_check, the 0 value propagates to l_dynaddr, and
>    "l_addr = l_dynaddr - dynaddr" then underflows, and things go downhill from
>    there, resulting in the warning and eventually the FAIL.
> 
> Fix this by replicating the missing glibc commit in
> svr4_solib_ops::read_lm_info.
> 
> I've enabled the fix only for the configuration I can test, for all others I
> disabled it using "lmo.l_real_offset = -1".
> 
> This is an RFC.  My question is: is the added complexity worth the trouble for
> what looks like a cornercase?
> 
> [ FTR, in an earlier attempt I proposed to deal with the FAIL using an
> xfail [1]. ]
> 

Ping.

Thanks,
- Tom

> Tested on ppc64le-linux and x86_64-linux.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33662
> 
> [1] https://sourceware.org/pipermail/gdb-patches/2026-July/229073.html
> ---
>   gdb/mips-fbsd-tdep.c   |  1 +
>   gdb/mips-netbsd-tdep.c |  1 +
>   gdb/solib-svr4-linux.c |  6 ++--
>   gdb/solib-svr4.c       | 67 +++++++++++++++++++++++++++++++-----------
>   gdb/solib-svr4.h       |  4 +++
>   5 files changed, 60 insertions(+), 19 deletions(-)
> 
> diff --git a/gdb/mips-fbsd-tdep.c b/gdb/mips-fbsd-tdep.c
> index ecad943e8fa..65278937415 100644
> --- a/gdb/mips-fbsd-tdep.c
> +++ b/gdb/mips-fbsd-tdep.c
> @@ -571,6 +571,7 @@ mips_fbsd_lp64_solib_ops::fetch_link_map_offsets () const
>         lmo.l_ld_offset = 24;
>         lmo.l_next_offset = 32;
>         lmo.l_prev_offset = 40;
> +      lmo.l_real_offset = -1;
>       }
>   
>     return lmp;
> diff --git a/gdb/mips-netbsd-tdep.c b/gdb/mips-netbsd-tdep.c
> index a5bcde20e46..79b1a3cedee 100644
> --- a/gdb/mips-netbsd-tdep.c
> +++ b/gdb/mips-netbsd-tdep.c
> @@ -333,6 +333,7 @@ mips_nbsd_ilp32_svr4_solib_ops::fetch_link_map_offsets () const
>         lmo.l_ld_offset = 12;
>         lmo.l_next_offset = 16;
>         lmo.l_prev_offset = 20;
> +      lmo.l_real_offset = -1;
>       }
>   
>     return lmp;
> diff --git a/gdb/solib-svr4-linux.c b/gdb/solib-svr4-linux.c
> index 2d36adcca73..e67f3036df7 100644
> --- a/gdb/solib-svr4-linux.c
> +++ b/gdb/solib-svr4-linux.c
> @@ -53,6 +53,7 @@ linux_ilp32_svr4_solib_ops::fetch_link_map_offsets () const
>         lmo.l_ld_offset = 8;
>         lmo.l_next_offset = 12;
>         lmo.l_prev_offset = 16;
> +      lmo.l_real_offset = -1;
>       }
>   
>     return lmp;
> @@ -85,13 +86,14 @@ linux_lp64_svr4_solib_ops::fetch_link_map_offsets () const
>         lmo.r_ldsomap_offset = -1;
>         lmo.r_next_offset = 40;
>   
> -      /* Everything we need is in the first 40 bytes.  */
> -      lmo.link_map_size = 40;
> +      /* Everything we need is in the first 48 bytes.  */
> +      lmo.link_map_size = 48;
>         lmo.l_addr_offset = 0;
>         lmo.l_name_offset = 8;
>         lmo.l_ld_offset = 16;
>         lmo.l_next_offset = 24;
>         lmo.l_prev_offset = 32;
> +      lmo.l_real_offset = 40;
>       }
>   
>     return lmp;
> diff --git a/gdb/solib-svr4.c b/gdb/solib-svr4.c
> index 8e3de4d3ea1..02a20f000d4 100644
> --- a/gdb/solib-svr4.c
> +++ b/gdb/solib-svr4.c
> @@ -189,27 +189,58 @@ svr4_solib_ops::read_lm_info (CORE_ADDR lm_addr, CORE_ADDR debug_base) const
>     gdb::byte_vector lm (lmo->link_map_size);
>   
>     if (target_read_memory (lm_addr, lm.data (), lmo->link_map_size) != 0)
> -    warning (_("Error reading shared library list entry at %s"),
> -	     paddress (current_inferior ()->arch (), lm_addr));
> -  else
>       {
> -      type *ptr_type
> -	= builtin_type (current_inferior ()->arch ())->builtin_data_ptr;
> +      warning (_("Error reading shared library list entry at %s"),
> +	       paddress (current_inferior ()->arch (), lm_addr));
> +      return lm_info;
> +    }
> +
> +  type *ptr_type
> +    = builtin_type (current_inferior ()->arch ())->builtin_data_ptr;
> +
> +  lm_info = std::make_unique<lm_info_svr4> (debug_base);
> +  lm_info->lm_addr = lm_addr;
>   
> -      lm_info = std::make_unique<lm_info_svr4> (debug_base);
> -      lm_info->lm_addr = lm_addr;
> -
> -      lm_info->l_addr_inferior = extract_typed_address (&lm[lmo->l_addr_offset],
> -							ptr_type);
> -      lm_info->l_ld = extract_typed_address (&lm[lmo->l_ld_offset], ptr_type);
> -      lm_info->l_next = extract_typed_address (&lm[lmo->l_next_offset],
> -					       ptr_type);
> -      lm_info->l_prev = extract_typed_address (&lm[lmo->l_prev_offset],
> -					       ptr_type);
> -      lm_info->l_name = extract_typed_address (&lm[lmo->l_name_offset],
> -					       ptr_type);
> +  lm_info->l_addr_inferior
> +    = extract_typed_address (&lm[lmo->l_addr_offset], ptr_type);
> +  lm_info->l_ld = extract_typed_address (&lm[lmo->l_ld_offset], ptr_type);
> +  lm_info->l_next = extract_typed_address (&lm[lmo->l_next_offset], ptr_type);
> +  lm_info->l_prev = extract_typed_address (&lm[lmo->l_prev_offset], ptr_type);
> +  lm_info->l_name = extract_typed_address (&lm[lmo->l_name_offset], ptr_type);
> +
> +  if (lmo->l_real_offset == -1)
> +    return lm_info;
> +  CORE_ADDR l_real = extract_typed_address (&lm[lmo->l_real_offset], ptr_type);
> +
> +  if (l_real == lm_addr)
> +    return lm_info;
> +
> +  /* We have l_real != lm_addr, so l_real points to the link map of ld.so.  */
> +
> +  if (!(lm_info->l_addr_inferior == 0 && lm_info->l_ld == 0))
> +    return lm_info;
> +
> +  /* We have l_addr_inferior == 0 and l_ld == 0.  This can happen with a glibc
> +     that:
> +     - has commit a93d9e03a3 ("Extend struct r_debug to support multiple
> +       namespaces [BZ #15971]"), but
> +     - misses commit 88361b408b ("elf: Copy l_addr/l_ld when adding ld.so to a
> +       new namespace").
> +     This seems to be the case at least for the alma linux 9.8 BaseOS version,
> +     which uses glibc v2.34 and backports only the first commit.
> +     Fix / work around this here by replicating the copy of ld_addr/l_ld.  */
> +
> +  if (target_read_memory (l_real, lm.data (), lmo->link_map_size) != 0)
> +    {
> +      warning (_("Error reading shared library list entry at %s"),
> +	       paddress (current_inferior ()->arch (), l_real));
> +      return lm_info;
>       }
>   
> +  lm_info->l_addr_inferior = extract_typed_address (&lm[lmo->l_addr_offset],
> +						    ptr_type);
> +  lm_info->l_ld = extract_typed_address (&lm[lmo->l_ld_offset], ptr_type);
> +
>     return lm_info;
>   }
>   
> @@ -3537,6 +3568,7 @@ ilp32_svr4_solib_ops::fetch_link_map_offsets () const
>         lmo.l_ld_offset = 8;
>         lmo.l_next_offset = 12;
>         lmo.l_prev_offset = 16;
> +      lmo.l_real_offset = -1;
>       }
>   
>     return lmp;
> @@ -3586,6 +3618,7 @@ lp64_svr4_solib_ops::fetch_link_map_offsets () const
>         lmo.l_ld_offset = 16;
>         lmo.l_next_offset = 24;
>         lmo.l_prev_offset = 32;
> +      lmo.l_real_offset = -1;
>       }
>   
>     return lmp;
> diff --git a/gdb/solib-svr4.h b/gdb/solib-svr4.h
> index 3078a092778..83d65842faa 100644
> --- a/gdb/solib-svr4.h
> +++ b/gdb/solib-svr4.h
> @@ -207,6 +207,10 @@ struct link_map_offsets
>   
>       /* Offset to l_name field in struct link_map.  */
>       int l_name_offset;
> +
> +    /* Offset to l_real field in struct link_map.  Available since glibc
> +       version v2.3.4.  */
> +    int l_real_offset;
>     };
>   
>   /* Set the gdbarch methods for SVR4 systems.  */
> 
> base-commit: e189bfd9b492a0361a37858b9a4709a52a92c803