Re: [PATCH] gdbserver: Handle DT_MIPS_RLD_MAP dynamic tag

Pedro Alves
2012-04-03T21:27:40Z

Hi Maciej,

On 04/03/2012 09:47 PM, Maciej W. Rozycki wrote:

> While working on a test case for an unrelated issue I have noticed
> single-stepping over system library calls is broken on the MIPS/Linux
> target when `gdbserver' is used. On closer inspection I have observed
> `gdbserver' reports no shared libraries loaded. Further investigation has
> revealed that the newly-added support for the `qXfer:libraries-svr4:read'
> packet does not handle the DT_MIPS_RLD_MAP dynamic tag and therefore
> cannot locate the link map on the MIPS target.

Yeah, I had noticed that on
<http://sourceware.org/ml/gdb-patches/2012-03/msg00142.html>, but never got
to propose a fix. Sorry about that.

> gdb-gdbserver-rld-map.diff

> Index: gdb-fsf-trunk-quilt/gdb/gdbserver/linux-low.c
> ===================================================================
> --- gdb-fsf-trunk-quilt.orig/gdb/gdbserver/linux-low.c 2012-04-03 13:38:58.575561428 +0100
> +++ gdb-fsf-trunk-quilt/gdb/gdbserver/linux-low.c 2012-04-03 13:39:17.315624148 +0100
> @@ -5419,7 +5419,9 @@ get_dynamic (const int pid, const int is
> }
>
> /* Return &_r_debug in the inferior, or -1 if not present. Return value
> - can be 0 if the inferior does not yet have the library list initialized. */
> + can be 0 if the inferior does not yet have the library list initialized.
> + We look for DT_MIPS_RLD_MAP first. MIPS executables use this instead of
> + DT_DEBUG, although they sometimes contain an unused DT_DEBUG entry too. */

>

Can that "unused DT_DEBUG" entry ever appear before DT_MIPS_RLD_MAP in the tags
list? With this patch, we'll stop looking if we see DT_DEBUG first, while
solib-svr4.c in GDB first looks for DT_MIPS_RLD_MAP in all tags, and only
if not found looks for DT_DEBUG (presumably, if there's no DT_MIPS_RLD_MAP,
then there's no DT_DEBUG either; only if DT_MIPS_RLD_MAP exist might there be
a DT_DEBUG). Do we need to worry about this?

> static CORE_ADDR
> get_r_debug (const int pid, const int is_elf64)
> @@ -5437,6 +5439,21 @@ get_r_debug (const int pid, const int is
> if (is_elf64)
> {
> Elf64_Dyn *const dyn = (Elf64_Dyn *) buf;
> + union
> + {
> + Elf64_Xword map;
> + unsigned char buf[sizeof (Elf64_Xword)];
> + }
> + rld_map;
> +
> + if (dyn->d_tag == DT_MIPS_RLD_MAP)
> + {
> + if (linux_read_memory (dyn->d_un.d_val,
> + rld_map.buf, sizeof (rld_map.buf)) == 0)
> + return rld_map.map;
> + else
> + break;
> + }
>
> if (dyn->d_tag == DT_DEBUG)
> return dyn->d_un.d_val;
> @@ -5447,6 +5464,21 @@ get_r_debug (const int pid, const int is
> else
> {
> Elf32_Dyn *const dyn = (Elf32_Dyn *) buf;
> + union
> + {
> + Elf32_Word map;
> + unsigned char buf[sizeof (Elf32_Word)];
> + }
> + rld_map;
> +
> + if (dyn->d_tag == DT_MIPS_RLD_MAP)
> + {
> + if (linux_read_memory (dyn->d_un.d_val,
> + rld_map.buf, sizeof (rld_map.buf)) == 0)
> + return rld_map.map;
> + else
> + break;
> + }
>
> if (dyn->d_tag == DT_DEBUG)
> return dyn->d_un.d_val;

--
Pedro Alves