[PATCH] Fix setting-breakpoints regression on PPC64 (function descriptors) (was: Re: ppc64 regression: [PATCH 1/2] Fix "list ambiguous_variable")

Pedro Alves
2017-11-29T19:08:02Z

On 11/26/2017 04:37 PM, Ulrich Weigand wrote:

> I now see this as well on Cell/B.E. This is a serious regression that
> causes "start" to always fail for me ...
>
> The problem seems to be that GDB sets a breakpoint into the function
> descriptor for main, which is not a good idea.
>
> Looking at the commit identified above, it seems that GDB now only
> runs the address through gdbarch_convert_from_func_ptr_addr if
> msymbol_is_text returns true. However, if the symbol points to
> a function descriptor, msymbol_is_text would be false since this
> is in fact outside the text section.
>
> So I think probably we need to still run the address through
> gdbarch_convert_from_func_ptr_addr, and if that detects that it
> was indeed a function descriptor, always treat the resulting
> address as a function.

Thanks for the suggestion. Here's a patch that implements
something like that. WDYT?

I've pushed this to the users/palves/fix-ppc64-func-descriptors
branch as well.

From d1e2a9dc61a645ff29606f01f96a02b477761c23 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Wed, 29 Nov 2017 18:06:33 +0000
Subject: [PATCH] Fix setting-breakpoints regression on PPC64 (function
descriptors)

The recent-ish commit e5f25bc5d6db ('Fix "list ambiguous_variable"')
caused a serious regression on PPC64. See
<https://sourceware.org/ml/gdb-patches/2017-11/msg00666.html>.

Basically, after that patch, GDB sets breakpoints in function
descriptors instead of where the descriptors point to, which is
incorrect.

The problem is that GDB now only runs a minsym's address through
gdbarch_convert_from_func_ptr_addr if msymbol_is_text returns true.
However, if the symbol points to a function descriptor,
msymbol_is_text is false since function descriptors are in fact
outside the text section.

The fix is to also run a non-text address through
gdbarch_convert_from_func_ptr_addr, and if that detects that it was
indeed a function descriptor, treat the resulting address as a
function.

While implementing that directly in linespec.c:minsym_found (where the
bad msymbol_is_text check is) fixes the issue, I noticed that
linespec.c:add_minsym has some code that also basically needs to do
the same checks, however it's implemented differently. Also,
add_minsym is calling find_pc_sect_line on non-function symbols, which
also doesn't look right.

So I introduced msymbol_is_function, so that we have a simple place to
consider minsyms and function descriptors.

And then, the only other use of msymbol_is_text is in
find_function_alias_target, which turns out to also be incorrect.
Changing that one to use msymbol_is_function, i.e., to consider
function descriptors too fixes (on PPC64):

-FAIL: gdb.base/symbol-alias.exp: p func_alias
-FAIL: gdb.base/symbol-alias.exp: p *func_alias()
+PASS: gdb.base/symbol-alias.exp: p func_alias
+PASS: gdb.base/symbol-alias.exp: p *func_alias()

And then after that, msymbol_is_text is no longer used anywhere, so it
can be removed.

Tested on x86_64 GNU/Linux, no regressions. Tested on PPC64 GNU/Linux
and results compared to a testrun of e5f25bc5d6db^ (before the
offending commit), also no regressions. (there's a couple new FAILs
and some new symbol name matching unit tests are crashing, but that
looks unrelated).

gdb/ChangeLog:
yyyy-mm-dd Pedro Alves <palves@redhat.com>

* linespec.c (minsym_found, add_minsym): Use msymbol_is_function.
* minsyms.c (msymbol_is_text): Delete.
(msymbol_is_function): New function.
* minsyms.h (msymbol_is_text): Delete.
(msymbol_is_function): New declaration.
* symtab.c (find_function_alias_target): Use msymbol_is_function.
---
gdb/linespec.c | 83 ++++++++++++++++------------------------------------------
gdb/minsyms.c | 34 ++++++++++++++++++------
gdb/minsyms.h | 11 +++++---
gdb/symtab.c | 8 +++---
4 files changed, 61 insertions(+), 75 deletions(-)

diff --git a/gdb/linespec.c b/gdb/linespec.c
index 3f7f171..83f9bbf 100644
--- a/gdb/linespec.c
+++ b/gdb/linespec.c
@@ -4309,22 +4309,16 @@ minsym_found (struct linespec_state *self, struct objfile *objfile,
struct minimal_symbol *msymbol,
std::vector<symtab_and_line> *result)
{
- struct gdbarch *gdbarch = get_objfile_arch (objfile);
- CORE_ADDR pc;
struct symtab_and_line sal;

- if (msymbol_is_text (msymbol))
- {
- sal = find_pc_sect_line (MSYMBOL_VALUE_ADDRESS (objfile, msymbol),
- (struct obj_section *) 0, 0);
- sal.section = MSYMBOL_OBJ_SECTION (objfile, msymbol);
+ /* The minimal symbol might point to a function descriptor, which is
+ not a text symbol; try resolving it to the actual code
+ address. */

- /* The minimal symbol might point to a function descriptor;
- resolve it to the actual code address instead. */
- pc = gdbarch_convert_from_func_ptr_addr (gdbarch, sal.pc,
- &current_target);
- if (pc != sal.pc)
- sal = find_pc_sect_line (pc, NULL, 0);
+ CORE_ADDR func_addr;
+ if (msymbol_is_function (objfile, msymbol, &func_addr))
+ {
+ sal = find_pc_sect_line (func_addr, NULL, 0);

if (self->funfirstline)
{
@@ -4332,14 +4326,9 @@ minsym_found (struct linespec_state *self, struct objfile *objfile,
&& (COMPUNIT_LOCATIONS_VALID (SYMTAB_COMPUNIT (sal.symtab))
|| SYMTAB_LANGUAGE (sal.symtab) == language_asm))
{
- /* If gdbarch_convert_from_func_ptr_addr does not apply then
- sal.SECTION, sal.LINE&co. will stay correct from above.
- If gdbarch_convert_from_func_ptr_addr applies then
- sal.SECTION is cleared from above and sal.LINE&co. will
- stay correct from the last find_pc_sect_line above. */
- sal.pc = MSYMBOL_VALUE_ADDRESS (objfile, msymbol);
- sal.pc = gdbarch_convert_from_func_ptr_addr (gdbarch, sal.pc,
- &current_target);
+ struct gdbarch *gdbarch = get_objfile_arch (objfile);
+
+ sal.pc = func_addr;
if (gdbarch_skip_entrypoint_p (gdbarch))
sal.pc = gdbarch_skip_entrypoint (gdbarch, sal.pc);
}
@@ -4424,52 +4413,26 @@ static void
add_minsym (struct minimal_symbol *minsym, void *d)
{
struct collect_minsyms *info = (struct collect_minsyms *) d;
- bound_minimal_symbol_d mo;
-
- mo.minsym = minsym;
- mo.objfile = info->objfile;

if (info->symtab != NULL)
{
- CORE_ADDR pc;
- struct symtab_and_line sal;
- struct gdbarch *gdbarch = get_objfile_arch (info->objfile);
-
- sal = find_pc_sect_line (MSYMBOL_VALUE_ADDRESS (info->objfile, minsym),
- NULL, 0);
- sal.section = MSYMBOL_OBJ_SECTION (info->objfile, minsym);
- pc
- = gdbarch_convert_from_func_ptr_addr (gdbarch, sal.pc, &current_target);
- if (pc != sal.pc)
- sal = find_pc_sect_line (pc, NULL, 0);
+ /* We're looking for a label for which we don't have debug
+ info. */
+ CORE_ADDR func_addr;
+ if (msymbol_is_function (info->objfile, minsym, &func_addr))
+ {
+ symtab_and_line sal = find_pc_sect_line (func_addr, NULL, 0);

- if (info->symtab != sal.symtab)
- return;
+ if (info->symtab != sal.symtab)
+ return;
+ }
}

- /* Exclude data symbols when looking for breakpoint locations. */
- if (!info->list_mode)
- switch (minsym->type)
- {
- case mst_slot_got_plt:
- case mst_data:
- case mst_bss:
- case mst_abs:
- case mst_file_data:
- case mst_file_bss:
- {
- /* Make sure this minsym is not a function descriptor
- before we decide to discard it. */
- struct gdbarch *gdbarch = get_objfile_arch (info->objfile);
- CORE_ADDR addr = gdbarch_convert_from_func_ptr_addr
- (gdbarch, BMSYMBOL_VALUE_ADDRESS (mo),
- &current_target);
-
- if (addr == BMSYMBOL_VALUE_ADDRESS (mo))
- return;
- }
- }
+ /* Exclude data symbols when looking for breakpoint locations. */
+ if (!info->list_mode && !msymbol_is_function (info->objfile, minsym))
+ return;

+ bound_minimal_symbol_d mo = {minsym, info->objfile};
VEC_safe_push (bound_minimal_symbol_d, info->msyms, &mo);
}

diff --git a/gdb/minsyms.c b/gdb/minsyms.c
index 4898da1..d68fb65 100644
--- a/gdb/minsyms.c
+++ b/gdb/minsyms.c
@@ -57,17 +57,35 @@
/* See minsyms.h. */

bool
-msymbol_is_text (minimal_symbol *msymbol)
+msymbol_is_function (struct objfile *objfile, minimal_symbol *minsym,
+ CORE_ADDR *func_address_p)
{
- switch (MSYMBOL_TYPE (msymbol))
+ CORE_ADDR msym_addr = MSYMBOL_VALUE_ADDRESS (objfile, minsym);
+
+ switch (minsym->type)
{
- case mst_text:
- case mst_text_gnu_ifunc:
- case mst_solib_trampoline:
- case mst_file_text:
- return true;
+ case mst_slot_got_plt:
+ case mst_data:
+ case mst_bss:
+ case mst_abs:
+ case mst_file_data:
+ case mst_file_bss:
+ {
+ struct gdbarch *gdbarch = get_objfile_arch (objfile);
+ CORE_ADDR pc = gdbarch_convert_from_func_ptr_addr (gdbarch, msym_addr,
+ &current_target);
+ if (pc != msym_addr)
+ {
+ if (func_address_p != NULL)
+ *func_address_p = pc;
+ return true;
+ }
+ return false;
+ }
default:
- return false;
+ if (func_address_p != NULL)
+ *func_address_p = msym_addr;
+ return true;
}
}

diff --git a/gdb/minsyms.h b/gdb/minsyms.h
index 5c0dde4..baa87f0 100644
--- a/gdb/minsyms.h
+++ b/gdb/minsyms.h
@@ -159,9 +159,14 @@ void terminate_minimal_symbol_table (struct objfile *objfile);

\f

-/* Return whether MSYMBOL is a function/method. */
-
-bool msymbol_is_text (minimal_symbol *msymbol);
+/* Return whether MSYMBOL is a function/method. If FUNC_ADDRESS_P is
+ non-NULL, and the MSYMBOL is a function, then *FUNC_ADDRESS_P is
+ set to the function's address, already resolved if MINSYM points to
+ a function descriptor. */
+
+bool msymbol_is_function (struct objfile *objfile,
+ minimal_symbol *minsym,
+ CORE_ADDR *func_address_p = NULL);

/* Compute a hash code for the string argument. */

diff --git a/gdb/symtab.c b/gdb/symtab.c
index 3d59367..a249a8d 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3927,14 +3927,14 @@ skip_prologue_using_sal (struct gdbarch *gdbarch, CORE_ADDR func_addr)
symbol *
find_function_alias_target (bound_minimal_symbol msymbol)
{
- if (!msymbol_is_text (msymbol.minsym))
+ CORE_ADDR func_addr;
+ if (!msymbol_is_function (msymbol.objfile, msymbol.minsym, &func_addr))
return NULL;

- CORE_ADDR addr = BMSYMBOL_VALUE_ADDRESS (msymbol);
- symbol *sym = find_pc_function (addr);
+ symbol *sym = find_pc_function (func_addr);
if (sym != NULL
&& SYMBOL_CLASS (sym) == LOC_BLOCK
- && BLOCK_START (SYMBOL_BLOCK_VALUE (sym)) == addr)
+ && BLOCK_START (SYMBOL_BLOCK_VALUE (sym)) == func_addr)
return sym;

return NULL;
--
2.5.5