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

Maciej W. Rozycki
2013-06-20T16:19:29Z

Tom,

I have now recalled that you made this comment about the original version
of this change:

On Thu, 21 Feb 2013, Tom Tromey wrote:

> Maciej> -/* In SVR4, we recognize a trampoline by it's section name.
> Maciej> - That is, if the pc is in a section named ".plt" then we are in
> Maciej> - a trampoline. */
> Maciej> +/* In SVR4, we recognize a trampoline by it's section name. That is,
> Maciej> + if the pc is in a section named ".plt" then we are in a trampoline.
> Maciej> + We let targets request an alternative name, this is currently used
> Maciej> + by the MIPS backend to handle the SVR4 lazy resolution stubs that
> Maciej> + binutils put into ".MIPS.stubs" instead. */
>
> IMO comments like this tend to get stale over time.
> It is better, I think, to describe the function's interface and purpose,
> and leave it to future developers to grep for uses of it, should they
> need to know.

I have now made the requested update, see below. Is this version OK to
apply?

2013-06-20 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 2013-06-19 16:54:49.000000000 +0100
+++ gdb-fsf-trunk-quilt/gdb/mips-linux-tdep.c 2013-06-19 16:55:00.280199593 +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 2013-06-19 16:54:49.000000000 +0100
+++ gdb-fsf-trunk-quilt/gdb/mips-tdep.c 2013-06-19 16:55:00.280199593 +0100
@@ -3628,12 +3628,7 @@ 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 2013-06-19 16:45:58.000000000 +0100
+++ gdb-fsf-trunk-quilt/gdb/objfiles.c 2013-06-20 17:10:59.240259104 +0100
@@ -1410,9 +1410,9 @@ 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. */
+/* Return non-zero if PC is in a dynamic function call trampoline. In
+ SVR4, we recognize a trampoline by its section name NAME, or ".plt"
+ if NAME is null. */

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