Re: [PATCH v4 1/2] Change calculation of frame_id by amd64 epilogue unwinder

[Simon Marchi <simark@simark.ca>]

On 10/25/22 09:44, Simon Marchi wrote:
> On 10/5/22 06:38, Bruno Larsen via Gdb-patches wrote:
>> When GDB is stopped at a ret instruction and no debug information is
>> available for unwinding, GDB defaults to the amd64 epilogue unwinder, to
>> be able to generate a decent backtrace. However, when calculating the
>> frame id, the epilogue unwinder generates information as if the return
>> instruction was the whole frame.
>>
>> This was an issue especially when attempting to reverse debug, as GDB
>> would place a step_resume_breakpoint from the epilogue of a function if
>> we were to attempt to skip that function, and this breakpoint should
>> ideally have the current function's frame_id to avoid other problems
>> such as PR record/16678.
>>
>> This commit changes the frame_id calculation for the amd64 epilogue,
>> so that it is always the same as the dwarf2 unwinder's frame_id.
>>
>> It also adds a test to confirm that the frame_id will be the same,
>> regardless of using the epilogue unwinder or not, thanks to Andrew
>> Burgess.
>>
>> Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
>> ---
>>  gdb/amd64-tdep.c                              |  10 +-
>>  .../gdb.base/unwind-on-each-insn-foo.c        |  22 +++
>>  gdb/testsuite/gdb.base/unwind-on-each-insn.c  |  25 +++
>>  .../gdb.base/unwind-on-each-insn.exp          | 154 ++++++++++++++++++/usr/lib/x86_64-linux-gnu/libasan.so.6
>>  4 files changed, 206 insertions(+), 5 deletions(-)
>>  create mode 100644 gdb/testsuite/gdb.base/unwind-on-each-insn-foo.c
>>  create mode 100644 gdb/testsuite/gdb.base/unwind-on-each-insn.c
>>  create mode 100644 gdb/testsuite/gdb.base/unwind-on-each-insn.exp
> 
> Hi Bruno,
> 
> On Ubuntu 22.04, I can get this new test to fail quite reliably with:
> 
>     $ taskset -c 1 make check TESTS="gdb.base/unwind-on-each-insn.exp" RUNTESTFLAGS="--target_board=native-gdbserver"
> 
> Can you give it a try?
> 
> Simon

Actually, I took the time to look into it, it turns out the problem is
simple.  Here's a patch below.


From 7090bf701b2f1cca89985ea1b45b0a2e3859e19e Mon Sep 17 00:00:00 2001
From: Simon Marchi <simon.marchi@efficios.com>
Date: Tue, 25 Oct 2022 09:50:56 -0400
Subject: [PATCH] gdb/testsuite: make sure to consume the prompt in
 gdb.base/unwind-on-each-insn.exp

This test fails quite reliably for me when ran as:

    $ taskset -c 1 make check TESTS="gdb.base/unwind-on-each-insn.exp" RUNTESTFLAGS="--target_board=native-gdbserver"

or more simply:

    $ make check-read1 TESTS="gdb.base/unwind-on-each-insn.exp"

The problem is that the that grabs the frame id from "maint print
frame-id" does not consume the prompt.  Well, it does sometimes due to
the trailing .*, but not always.  If the prompt is not consumed, the
following tests get confused:

    FAIL: gdb.base/unwind-on-each-insn.exp: gdb_breakpoint: set breakpoint at *foo
    FAIL: gdb.base/unwind-on-each-insn.exp: disassemble foo
    FAIL: gdb.base/unwind-on-each-insn.exp: get $sp and frame base in foo: get hexadecimal valueof "$sp"

Use -wrap to make gdb_test_multiple consume the prompt.

While at it, remove the bit that consumes the command name and do
exp_continue, it's not really necessary.  And for consistency, do the
same changes to the gdb_test_multiple that consumes the stack address,
although that one was fine, it did consume the prompt explicitly.

Change-Id: I2b7328c8844c7e98921ea494c4c05107162619fc
---
 gdb/testsuite/gdb.base/unwind-on-each-insn.exp | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/gdb/testsuite/gdb.base/unwind-on-each-insn.exp b/gdb/testsuite/gdb.base/unwind-on-each-insn.exp
index faa6a1a3f064..3b48805cff83 100644
--- a/gdb/testsuite/gdb.base/unwind-on-each-insn.exp
+++ b/gdb/testsuite/gdb.base/unwind-on-each-insn.exp
@@ -44,11 +44,7 @@ proc get_sp_and_fba { testname } {

 	set fba ""
 	gdb_test_multiple "info frame" "" {
-	    -re "^info frame\r\n" {
-		exp_continue
-	    }
-
-	    -re "^Stack level ${::decimal}, frame at ($::hex):\r\n.*$::gdb_prompt $" {
+	    -re -wrap ".*Stack level ${::decimal}, frame at ($::hex):.*" {
 		set fba $expect_out(1,string)
 	    }
 	}
@@ -62,11 +58,7 @@ proc get_sp_and_fba { testname } {
 proc get_fid { } {
     set fid ""
     gdb_test_multiple "maint print frame-id" "" {
-	-re "^maint print frame-id\r\n" {
-	    exp_continue
-	}
-
-	-re "^frame-id for frame #${::decimal}: (\[^\r\n\]+).*" {
+	-re -wrap ".*frame-id for frame #${::decimal}: (.*)" {
 	    set fid $expect_out(1,string)
 	}
     }
-- 
2.38.0