[PATCH] in_plt_section: support alternate stub section names (was: [PATCH 1/2] MIPS: Compressed PLT/stubs support)

Maciej W. Rozycki
2013-06-07T13:25:56Z

Hi,

I have realised the change to support alternate stub section names in
in_plt_section is really self-contained and while a prerequisite for
microMIPS/MIPS16 PLT support it can be applied separately. I have
therefore split it off from the PLT change, hopefully making a review
easier.

For a reference, here are the relevant original observations I made when
posting the combined change:

> As to the semantics change of the in_plt_section GDB helper -- the `name'
> argument is unused and all the callers pass it as NULL. I've tracked down
> the history of this function, and it was introduced with GDB 4.13:
>
> Fri Apr 1 00:44:00 1994 Peter Schauer (pes@regent.e-technik.tu-muenchen.de)
>
> * sparc-tdep.c (in_solib_trampoline): Renamed to in_plt_section
> and moved to objfiles.c.
> * objfiles.c (in_plt_section): Moved to here from sparc-tdep.
> * config/tm-sysv4.h (IN_SOLIB_TRAMPOLINE): Use new in_plt_section.
> * config/sparc/tm-sun4sol2.h (IN_SOLIB_TRAMPOLINE): Removed,
> the new generic definition from tm-sysv4.h works for Solaris.
>
> -- with this argument already unused. Furthermore, `in_solib_trampoline'
> was introduced in GDB 4.9:
>
> Tue Mar 30 15:46:14 1993 K. Richard Pixley (rich@rtl.cygnus.com)
>
> * sparc-tdep.c (in_solib_trampoline): new function.
> * config/sparc/tm-sun4sol2.h (IN_SOLIB_TRAMPOLINE): redefine to
> in_solib_trampoline.
>
> with this argument also unused. I was unable to track down the pre-4.9
> tm-sun4sol2.h version of IN_SOLIB_TRAMPOLINE as GDB 4.8 didn't have the
> macro there yet, so no GDB version was ever released that provided it.
>
> However, the tm-sysv4.h version was defined like this:
>
> #define IN_SOLIB_TRAMPOLINE(pc,name) ((name) && (STREQ ("_init", name)))
>
> -- and then redefined in terms of in_plt_section as recorded in the
> ChangeLog entry quoted above like this:
>
> #define IN_SOLIB_TRAMPOLINE(pc, name) in_plt_section((pc), (name))
>
> at which point the `name' argument became unused as well.
>
> HP-PA had its own version:
>
> #define IN_SOLIB_TRAMPOLINE(pc, name) skip_trampoline_code (pc, name)
>
> -- but skip_trampoline_code didn't make any use of its `name' argument
> either -- just as does't current code in hppa_in_solib_call_trampoline the
> former has evolved to (and neither does code in
> hppa32_hpux_in_solib_call_trampoline, hppa64_hpux_in_solib_call_trampoline
> or hppa_hpux_in_solib_return_trampoline).
>
> With the above consideration in mind, I think it is safe to redefine
> in_plt_section's API as proposed in this change -- remembering that MIPS
> stubs are the functional equivalent of PLT entries -- for the sake of code
> duplication avoidance.

With in_plt_section such redefined, all the handcoded conditions
throughout the MIPS backend can be unified, and also the helper can now be
used in mips_linux_in_dynsym_stub to avoid the heuristic there if
possible.

This change was regression-tested for the mips-sde-elf and mips-linux-gnu
targets using the following configurations (multilibs), both endiannesses
each:

* o32 (-mabi=32),

* n64 (-mabi=64) (mips-linux-gnu only),

* n32 (-mabi=n32) (mips-linux-gnu only),

* MIPS16 o32 (-mips16 -mabi=32),

* microMIPS o32 (-mmicromips -mabi=32).

with no regressions (as previously, with the outstanding ISA bit fix
applied). OK to apply?

2013-06-07 Maciej W. Rozycki <macro@codesourcery.com>

gdb/
* mips-linux-tdep.c (mips_linux_in_dynsym_stub): Handle
.MIPS.stubs section like .plt. Remove unused `name' argument.
Return 1 rather than the low 16-bit halfword of any instruction
examined.
(mips_linux_in_dynsym_resolve_code): Update accordingly.
* mips-tdep.c (mips_stub_frame_sniffer): Call in_plt_section in
place of an equivalent hand-coded sequence.
* objfiles.c (in_plt_section): Reuse the `name' argument as an
trampoline section name override.
Maciej

gdb-mips-in-stubs-section.diff
Index: gdb-fsf-trunk-quilt/gdb/mips-linux-tdep.c
===================================================================
--- gdb-fsf-trunk-quilt.orig/gdb/mips-linux-tdep.c 2013-06-06 20:48:30.243223201 +0100
+++ gdb-fsf-trunk-quilt/gdb/mips-linux-tdep.c 2013-06-06 20:52:00.273227140 +0100
@@ -30,6 +30,7 @@
#include "trad-frame.h"
#include "tramp-frame.h"
#include "gdbtypes.h"
+#include "objfiles.h"
#include "solib.h"
#include "solib-svr4.h"
#include "solist.h"
@@ -666,25 +667,34 @@ mips_linux_core_read_description (struct

/* Check the code at PC for a dynamic linker lazy resolution stub.
- Because they aren't in the .plt section, we pattern-match on the
- code generated by GNU ld. They look like this:
+ GNU ld for MIPS has put lazy resolution stubs into a ".MIPS.stubs"
+ section uniformly since version 2.15. If the pc is in that section,
+ then we are in such a stub. Before that ".stub" was used in 32-bit
+ ELF binaries, however we do not bother checking for that since we
+ have never had and that case should be extremely rare these days.
+ Instead we pattern-match on the code generated by GNU ld. They look
+ like this:

lw t9,0x8010(gp)
addu t7,ra
jalr t9,ra
addiu t8,zero,INDEX

- (with the appropriate doubleword instructions for N64). Also
- return the dynamic symbol index used in the last instruction. */
+ (with the appropriate doubleword instructions for N64). As any lazy
+ resolution stubs in microMIPS binaries will always be in a
+ ".MIPS.stubs" section we only ever verify standard MIPS patterns. */

static int
-mips_linux_in_dynsym_stub (CORE_ADDR pc, char *name)
+mips_linux_in_dynsym_stub (CORE_ADDR pc)
{
gdb_byte buf[28], *p;
ULONGEST insn, insn1;
int n64 = (mips_abi (target_gdbarch ()) == MIPS_ABI_N64);
enum bfd_endian byte_order = gdbarch_byte_order (target_gdbarch ());

+ if (in_plt_section (pc, ".MIPS.stubs"))
+ return 1;
+
read_memory (pc - 12, buf, 28);

if (n64)
@@ -742,7 +752,7 @@ mips_linux_in_dynsym_stub (CORE_ADDR pc,
return 0;
}

- return (insn & 0xffff);
+ return 1;
}

/* Return non-zero iff PC belongs to the dynamic linker resolution
@@ -756,9 +766,10 @@ mips_linux_in_dynsym_resolve_code (CORE_
if (svr4_in_dynsym_resolve_code (pc))
return 1;

- /* Pattern match for the stub. It would be nice if there were a
- more efficient way to avoid this check. */
- if (mips_linux_in_dynsym_stub (pc, NULL))
+ /* Likewise for the stubs. They live in the .MIPS.stubs section these
+ days, so we check if the PC is within, than fall back to a pattern
+ match. */
+ if (mips_linux_in_dynsym_stub (pc))
return 1;

return 0;
Index: gdb-fsf-trunk-quilt/gdb/mips-tdep.c
===================================================================
--- gdb-fsf-trunk-quilt.orig/gdb/mips-tdep.c 2013-06-06 20:48:30.243223201 +0100
+++ gdb-fsf-trunk-quilt/gdb/mips-tdep.c 2013-06-06 20:52:00.273227140 +0100
@@ -3591,12 +3591,7 @@ mips_stub_frame_sniffer (const struct fr
if (in_plt_section (pc, NULL))
return 1;

- /* Binutils for MIPS puts lazy resolution stubs into .MIPS.stubs. */
- s = find_pc_section (pc);
-
- if (s != NULL
- && strcmp (bfd_get_section_name (s->objfile->obfd, s->the_bfd_section),
- ".MIPS.stubs") == 0)
+ if (in_plt_section (pc, ".MIPS.stubs"))
return 1;

/* Calling a PIC function from a non-PIC function passes through a
Index: gdb-fsf-trunk-quilt/gdb/objfiles.c
===================================================================
--- gdb-fsf-trunk-quilt.orig/gdb/objfiles.c 2013-06-06 20:48:30.243223201 +0100
+++ gdb-fsf-trunk-quilt/gdb/objfiles.c 2013-06-06 20:52:00.273227140 +0100
@@ -1410,9 +1410,11 @@ find_pc_section (CORE_ADDR pc)
}

-/* In SVR4, we recognize a trampoline by it's section name.
- That is, if the pc is in a section named ".plt" then we are in
- a trampoline. */
+/* In SVR4, we recognize a trampoline by it's section name. That is,
+ if the pc is in a section named ".plt" then we are in a trampoline.
+ We let targets request an alternative name, this is currently used
+ by the MIPS backend to handle the SVR4 lazy resolution stubs that
+ binutils put into ".MIPS.stubs" instead. */

int
in_plt_section (CORE_ADDR pc, char *name)
@@ -1424,7 +1426,7 @@ in_plt_section (CORE_ADDR pc, char *name

retval = (s != NULL
&& s->the_bfd_section->name != NULL
- && strcmp (s->the_bfd_section->name, ".plt") == 0);
+ && strcmp (s->the_bfd_section->name, name ? name : ".plt") == 0);
return (retval);
}
\f