Re: [PATCH] gdbserver: Handle DT_MIPS_RLD_MAP dynamic tag

Maciej W. Rozycki
2012-04-04T11:37:10Z

Hi Pedro,

> 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.

Well, I'm supposed to catch this kind of breakage myself these days. ;)

> > 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?

Hmm, I've double-checked the sources and as you say solib-svr4.c does not
rely on the order of the tags indeed. I got mislead by the comment in
MIPS BFD that says:

/* SGI object has the equivalence of DT_DEBUG in the
DT_MIPS_RLD_MAP entry. This must come first because glibc
only fills in DT_MIPS_RLD_MAP (not DT_DEBUG) and GDB only
looks at the first one it sees. */

and which obviously does not stand (anymore?) as far as GDB is concerned.
I'll post a proposal to update it to match reality. It is worth noting
that current MIPS BFD always adds DT_DEBUG too (and always after
DT_MIPS_RLD_MAP, the comment gets this part right).

Anyway, I think `gdbserver' should be as liberal as to what it accepts as
GDB itself is, so I have updated my change as below. No changes in
regression testing.

Thanks for your input. Any further comments?

2012-04-03 Maciej W. Rozycki <macro@codesourcery.com>

gdb/gdbserver/
* linux-low.c (get_r_debug): Handle DT_MIPS_RLD_MAP.

Maciej

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 16:49:32.000000000 +0100
+++ gdb-fsf-trunk-quilt/gdb/gdbserver/linux-low.c 2012-04-04 11:56:37.405566578 +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. */

static CORE_ADDR
get_r_debug (const int pid, const int is_elf64)
@@ -5427,19 +5429,35 @@ get_r_debug (const int pid, const int is
CORE_ADDR dynamic_memaddr;
const int dyn_size = is_elf64 ? sizeof (Elf64_Dyn) : sizeof (Elf32_Dyn);
unsigned char buf[sizeof (Elf64_Dyn)]; /* The larger of the two. */
+ CORE_ADDR map = -1;

dynamic_memaddr = get_dynamic (pid, is_elf64);
if (dynamic_memaddr == 0)
- return (CORE_ADDR) -1;
+ return map;

while (linux_read_memory (dynamic_memaddr, buf, dyn_size) == 0)
{
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_DEBUG)
- return dyn->d_un.d_val;
+ 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 && map == -1)
+ map = dyn->d_un.d_val;

if (dyn->d_tag == DT_NULL)
break;
@@ -5447,9 +5465,24 @@ 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_DEBUG)
- return dyn->d_un.d_val;
+ 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 && map == -1)
+ map = dyn->d_un.d_val;

if (dyn->d_tag == DT_NULL)
break;
@@ -5458,7 +5491,7 @@ get_r_debug (const int pid, const int is
dynamic_memaddr += dyn_size;
}

- return (CORE_ADDR) -1;
+ return map;
}

/* Read one pointer from MEMADDR in the inferior. */