Re: [pushed] Change inline frame breakpoint skipping logic (fix gdb.gdb/selftest.exp)

Pedro Alves
2018-06-26T19:02:07Z

On 06/25/2018 10:04 PM, Joel Brobecker wrote:
> Hello,
>
>> gdb/ChangeLog:
>> 2018-06-19 Pedro Alves <palves@redhat.com>
>>
>> * inline-frame.c (stopped_by_user_bp_inline_frame): Replace PC
>> parameter with a block parameter. Compare location's block symbol
>> with the frame's block instead of addresses.
>> (skip_inline_frames): Pass the current block instead of the
>> frame's address. Break out as soon as we determine the frame
>> should not be skipped.
>>
>> gdb/testsuite/ChangeLog:
>> 2018-06-19 Pedro Alves <palves@redhat.com>
>>
>> * gdb.opt/inline-break.c (func_inline_callee, func_inline_caller)
>> (func_extern_caller): New.
>> (main): Call func_extern_caller.
>> * gdb.opt/inline-break.exp: Add tests for inline frame skipping
>> logic change.
>
> it looks like this patch is causing a crash with the following
> example program:
>
> $ cat -n r.h
> 1 /* r.h */
> 2 int counter = 42;
> 3
> 4 inline void
> 5 callee () {
> 6 counter = 0; /* break here */
> 7 }
> $ cat -n r.c
> 1 /* r.c */
> 2 #include "r.h"
> 3
> 4 int
> 5 main ()
> 6 {
> 7 callee ();
> 8 }
>
> I compiled it using the following commands:
>
> $ gcc -c -g -O2 r.c
> $ gcc -o r r.o
>
> Then, trying to put a breakpoint on r.h:6 (inside "callee") causes
> a SEGV for me:
>
> $ gdb -q r
> Reading symbols from r...done.
> (gdb) b r.h:6
> Breakpoint 1 at 0x4003c0: file r.h, line 6.
> (gdb) run
> Starting program: /[...]/r
> [1] 75618 segmentation fault /[...]/gdb -q r
>
> Prior to this commit, the behavior is the following for the "run"
> command:
>
> (gdb) run
> Starting program: /[...]/r
>
> Breakpoint 1, callee () at r.h:6
> 6 counter = 0; /* break here */
>
> The problem occurs because we apparently assume that a bp_location's
> symbols is not NULL:
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x00000000006f42bb in stopped_by_user_bp_inline_frame (
> stop_chain=<optimized out>, frame_block=<optimized out>)
> at /homes/brobecke/act/gdb/gdb-head/gdb/inline-frame.c:305
> 305 && frame_block == SYMBOL_BLOCK_VALUE (loc->symbol))
> (gdb) p loc->symbol
> $1 = (const symbol *) 0x0

Whoops. I remember actually thinking about loc->symbol potentially
being null, but somehow convinced myself that SYMBOL_BLOCK_VALUE
would return null in that case... :-P

>
> I don't know yet whether that's a valid assumption or something
> occurred earlier in the process. Any thoughts on this before I start
> looking deeper?
>
> I'm using a version of GCC 7.3.1 on x86_64-linux if anyone wants to
> reproduce.

If we just add a "loc->symbol != NULL" check, then we end up
presenting the stop at the caller of the inline function, where
the inline function was inlined, which is not what we want, since that's
not where the user set the breakpoint.

Recording the symbol in the location (it is copied from the sal
that linespec.c creates into the location by
add_location_to_breakpoint), like in the patch below, makes gdb present
the stop at the "break here" line successfully.

I tried to come up with a more complicated testcase that included
more nested blocks inside the inlined function, to see if we would
need to record the inner inlined block in the sal instead of the
function's symbol (does that actually make sense for inlined functions?),
but all I came up with worked with this patch as is. So maybe we
can defer thinking about that until we find a testcase?

From ac746927c3a8078098729dc3256010b2c1e617f8 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Tue, 26 Jun 2018 19:14:41 +0100
Subject: [PATCH] inline

---
gdb/inline-frame.c | 1 +
gdb/linespec.c | 1 +
2 files changed, 2 insertions(+)

diff --git a/gdb/inline-frame.c b/gdb/inline-frame.c
index 896b0004e4a..3d07f8d0970 100644
--- a/gdb/inline-frame.c
+++ b/gdb/inline-frame.c
@@ -302,6 +302,7 @@ stopped_by_user_bp_inline_frame (const block *frame_block, bpstat stop_chain)

if ((t == bp_loc_software_breakpoint
|| t == bp_loc_hardware_breakpoint)
+ && loc->symbol != nullptr
&& frame_block == SYMBOL_BLOCK_VALUE (loc->symbol))
return true;
}
diff --git a/gdb/linespec.c b/gdb/linespec.c
index ae0200b8133..93e66c389f7 100644
--- a/gdb/linespec.c
+++ b/gdb/linespec.c
@@ -2196,6 +2196,7 @@ create_sals_line_offset (struct linespec_state *self,

if (self->funfirstline)
skip_prologue_sal (&intermediate_results[i]);
+ intermediate_results[i].symbol = sym;
add_sal_to_sals (self, &values, &intermediate_results[i],
sym ? SYMBOL_NATURAL_NAME (sym) : NULL, 0);
}
--
2.14.4

Thanks,
Pedro Alves