[pushed] Change inline frame breakpoint skipping logic (fix gdb.gdb/selftest.exp)
Commit Message
On 06/14/2018 02:22 PM, Tom de Vries wrote:
> On 06/12/2018 07:38 PM, Pedro Alves wrote:
>> Yes, sounds like it. But the selftest.exp explicitly asks to stop
>> at "captured_main", not "captured_main_1", so I'm thinking that
>> it's gdb's behavior that might be wrong:
>>
> Agreed, that's a better solution.
>
>> Note that both captured_main and captured_main_1 resolved to the
>> same address, 0x791339.
>
> Right. I played around a bit with this, and set breakpoints on
> captured_main and captured_main_1.
>
> If I set a breakpoint on captured_main_1, we have captured_main unknown:
> ...
> Breakpoint 2, captured_main_1 (context=<optimized out>)
> at /home/vries/gdb_versions/devel/src/gdb/main.c:492
> 492 lim_at_start = (char *) sbrk (0);
> (gdb) p captured_main
> No symbol "captured_main" in current context.
> (gdb) p captured_main_1
> $1 = {void (captured_main_args *)} 0x61b959
> <gdb_main(captured_main_args*)+25>
> ...
>
> But If I set a breakpoint on captured_main instead, we have
> captured_main_1 unknown:
> ...
> Breakpoint 3, captured_main (data=<optimized out>)
> at /home/vries/gdb_versions/devel/src/gdb/main.c:1147
> 1147 captured_main_1 (context);
> (gdb) p captured_main
> $2 = {void (void *)} 0x61b959 <gdb_main(captured_main_args*)+25>
> (gdb) p captured_main_1
> No symbol "captured_main_1" in current context.
> ...
>
> And if I set a breakpoint on both, captured_main_1 seems to take
> precedence (independent of the order used to set the breakpoint):
> ...
> Breakpoint 1, captured_main_1 (context=<optimized out>)
> at /home/vries/gdb_versions/devel/src/gdb/main.c:492
> 492 lim_at_start = (char *) sbrk (0);
> (gdb) p captured_main_1
> $1 = {void (captured_main_args *)} 0x61b959
> <gdb_main(captured_main_args*)+25>
> (gdb) p captured_main
> No symbol "captured_main" in current context.
> ...
>
> I don't understand the underlying mechanisms well enough to decide
> whether this is a problem or not, but I thought I just mention it.
Can't pinpoint offhand where the problem is, but sounds like
something in the dwarf or elf symbol readers, maybe the breakpoint
you set first changes the order in which symbols are read and are
added to search hashes etc., or something like that.
It's most certainly unrelated to this change though.
>
>> The gdb.base/inline-break.exp testcase
>> currently does not exercise that, but the new test added by the
>> patch below does. That new test fails without the patch and passes
>> with the patch. No regressions on x86-64 GNU/Linux. WDYT?
>>
>
> AFAICT, the patch looks ok (just one nit below).
>
>> +/* A static inlined function that is called by another static inlined
>> + function. */
>> +
>> +static inline ATTR int
>> +func_callee (int x)
>> +{
>> + return x * 23;
>> +}
>> +
>> +/* A static inlined function that calls another static inlined
>> + function. The body of the function is a simple as possible so that
>> + both functions are inlined to the same PC address. */
>> +
>> +static int
>
> inline ATTR ?
Hmm, indeed. If I do that however gcc (7.3) seemingly optimizes out
the functions more aggressively and we can't set a breakpoint anymore:
(gdb) b func_inline_caller
Function "func_inline_caller" not defined.
Breakpoint 1 (func_inline_caller) pending.
A quick look at the debug info reveals that the debug info
does mention the functions, so this may be another gdb bug:
<1><2c3>: Abbrev Number: 12 (DW_TAG_subprogram)
<2c4> DW_AT_name : (indirect string, offset: 0x16f): func_inline_caller
<2c8> DW_AT_decl_file : 1
<2c9> DW_AT_decl_line : 197
<2ca> DW_AT_prototyped : 1
<2ca> DW_AT_type : <0x2a4>
<2ce> DW_AT_inline : 3 (declared as inline and inlined)
<2cf> DW_AT_sibling : <0x2dd>
...
<1><2dd>: Abbrev Number: 12 (DW_TAG_subprogram)
<2de> DW_AT_name : (indirect string, offset: 0x112): func_inline_callee
<2e2> DW_AT_decl_file : 1
<2e3> DW_AT_decl_line : 187
<2e4> DW_AT_prototyped : 1
<2e4> DW_AT_type : <0x2a4>
<2e8> DW_AT_inline : 3 (declared as inline and inlined)
<2e9> DW_AT_sibling : <0x2f7>
I haven't investigated that one.
In order to move forward with the frame skipping patch, I'm
adding a non-inline caller level in the testcase instead.
Below's what I've now merged.
From 4de1f9e76460db0f6b97762ff368e5b8f0da16b0 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Tue, 19 Jun 2018 16:30:13 +0100
Subject: [PATCH] Change inline frame breakpoint skipping logic (fix
gdb.gdb/selftest.exp)
Currently, gdb.gdb/selftest.exp fails if you build GDB with
optimization (-O2, etc.).
The reason is that after setting a breakpoint in captured_main, we
stop at:
...
Breakpoint 1, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
...
while selftest_setup expects a stop at captured_main.
Here, captured_main_1 has been inlined into captured_main, and
captured_main has been inlined into gdb_main:
...
$ nm ./build/gdb/gdb | egrep ' [tT] .*captured_main|gdb_main' | c++filt
000000000061b950 T gdb_main(captured_main_args*)
...
Indeed, the two inlined functions show up in the backtrace:
...
(gdb) bt
#0 captured_main_1 (context=<optimized out>) at main.c:492
#1 captured_main (data=<optimized out>) at main.c:1147
#2 gdb_main (args=args@entry=0x7fffffffdb80) at main.c:1173
#3 0x000000000040fea5 in main (argc=<optimized out>, argv=<optimized out>)
at gdb.c:32
...
We're now stopping at captured_main_1 because commit ddfe970e6bec
("Don't elide all inlined frames") makes GDB present a stop at the
innermost inlined frame if the program stopped by a user breakpoint.
Now, the selftest.exp testcase explicitly asks to stop at
"captured_main", not "captured_main_1", so I'm thinking that it's
GDB'S behavior that should be improved. That is what this commit
does, by only showing a stop at an inline frame if the user breakpoint
was set in that frame's block.
Before this commit:
(top-gdb) b captured_main
Breakpoint 1 at 0x792f99: file src/gdb/main.c, line 492.
(top-gdb) r
Starting program: build/gdb/gdb
Breakpoint 1, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
492 lim_at_start = (char *) sbrk (0);
(top-gdb)
After this commit, we now instead get:
(top-gdb) b captured_main
Breakpoint 1 at 0x791339: file src/gdb/main.c, line 492.
(top-gdb) r
Starting program: build/gdb/gdb
Breakpoint 1, captured_main (data=<optimized out>) at src/gdb/main.c:1147
1147 captured_main_1 (context);
(top-gdb)
and:
(top-gdb) b captured_main_1
Breakpoint 2 at 0x791339: file src/gdb/main.c, line 492.
(top-gdb) r
Starting program: build/gdb/gdb
Breakpoint 2, captured_main_1 (context=<optimized out>) at src/gdb/main.c:492
492 lim_at_start = (char *) sbrk (0);
(top-gdb)
Note that both captured_main and captured_main_1 resolved to the same
address, 0x791339. That is necessary to trigger the issue in
question. The gdb.base/inline-break.exp testcase currently does not
exercise that, but the new test added by this commit does. That new
test fails without the GDB fix and passes with the fix. No
regressions on x86-64 GNU/Linux.
While at it, the THIS_PC comparison in stopped_by_user_bp_inline_frame
is basically a nop, so just remove it -- if a software or hardware
breakpoint explains the stop, then it must be that it was installed at
the current PC.
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.
---
gdb/inline-frame.c | 23 +++++++++++------------
gdb/testsuite/gdb.opt/inline-break.c | 34 ++++++++++++++++++++++++++++++++++
gdb/testsuite/gdb.opt/inline-break.exp | 25 +++++++++++++++++++++++++
3 files changed, 70 insertions(+), 12 deletions(-)
Comments
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
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.
@@ -286,11 +286,10 @@ block_starting_point_at (CORE_ADDR pc, const struct block *block)
}
/* Loop over the stop chain and determine if execution stopped in an
- inlined frame because of a user breakpoint. THIS_PC is the current
- frame's PC. */
+ inlined frame because of a user breakpoint set at FRAME_BLOCK. */
static bool
-stopped_by_user_bp_inline_frame (CORE_ADDR this_pc, bpstat stop_chain)
+stopped_by_user_bp_inline_frame (const block *frame_block, bpstat stop_chain)
{
for (bpstat s = stop_chain; s != NULL; s = s->next)
{
@@ -301,9 +300,9 @@ stopped_by_user_bp_inline_frame (CORE_ADDR this_pc, bpstat stop_chain)
bp_location *loc = s->bp_location_at;
enum bp_loc_type t = loc->loc_type;
- if (loc->address == this_pc
- && (t == bp_loc_software_breakpoint
- || t == bp_loc_hardware_breakpoint))
+ if ((t == bp_loc_software_breakpoint
+ || t == bp_loc_hardware_breakpoint)
+ && frame_block == SYMBOL_BLOCK_VALUE (loc->symbol))
return true;
}
}
@@ -340,12 +339,12 @@ skip_inline_frames (ptid_t ptid, bpstat stop_chain)
{
/* Do not skip the inlined frame if execution
stopped in an inlined frame because of a user
- breakpoint. */
- if (!stopped_by_user_bp_inline_frame (this_pc, stop_chain))
- {
- skip_count++;
- last_sym = BLOCK_FUNCTION (cur_block);
- }
+ breakpoint for this inline function. */
+ if (stopped_by_user_bp_inline_frame (cur_block, stop_chain))
+ break;
+
+ skip_count++;
+ last_sym = BLOCK_FUNCTION (cur_block);
}
else
break;
@@ -176,6 +176,38 @@ not_inline_func3 (int x)
return y + inline_func3 (x);
}
+/* The following three functions serve to exercise GDB's inline frame
+ skipping logic when setting a user breakpoint on an inline function
+ by name. */
+
+/* A static inlined function that is called by another static inlined
+ function. */
+
+static inline ATTR int
+func_inline_callee (int x)
+{
+ return x * 23;
+}
+
+/* A static inlined function that calls another static inlined
+ function. The body of the function is as simple as possible so
+ that both functions are inlined to the same PC address. */
+
+static inline ATTR int
+func_inline_caller (int x)
+{
+ return func_inline_callee (x);
+}
+
+/* An extern not-inline function that calls a static inlined
+ function. */
+
+int
+func_extern_caller (int x)
+{
+ return func_inline_caller (x);
+}
+
/* Entry point. */
int
@@ -205,5 +237,7 @@ main (int argc, char *argv[])
x = not_inline_func3 (-21);
+ func_extern_caller (1);
+
return x;
}
@@ -231,4 +231,29 @@ foreach_with_prefix cmd [list "break" "tbreak"] {
}
}
+# func_extern_caller calls func_inline_caller which calls
+# func_inline_callee. The latter two are both inline functions. Test
+# that setting a breakpoint on each of the functions reports a stop at
+# that function. This exercises the inline frame skipping logic. If
+# we set a breakpoint at function A, we want to present the stop at A,
+# even if A's entry code is an inlined call to another inline function
+# B.
+
+foreach_with_prefix func {
+ "func_extern_caller"
+ "func_inline_caller"
+ "func_inline_callee"
+} {
+ clean_restart $binfile
+
+ if {![runto main]} {
+ untested "could not run to main"
+ continue
+ }
+
+ gdb_breakpoint $func
+ gdb_test "continue" "Breakpoint .* $func .*at .*$srcfile.*" \
+ "breakpoint hit presents stop at breakpointed function"
+}
+
unset -nocomplain results