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[PATCH] gdb: make gdb.arch/amd64-disp-step-avx.exp actually test displaced stepping
- From: Simon Marchi <simon dot marchi at efficios dot com>
- To: gdb-patches at sourceware dot org
- Cc: Simon Marchi <simon dot marchi at efficios dot com>
- Date: Thu, 5 Mar 2020 17:21:08 -0500
- Subject: [PATCH] gdb: make gdb.arch/amd64-disp-step-avx.exp actually test displaced stepping
The test gdb.arch/amd64-disp-step-avx.exp is meant to test that doing a
displaced step of an AVX instruction works correctly. However, I found
(by pure coincidence) that the test instructions are not actually
displaced stepped. Rather, they are inline-stepped, so the test is not
actually testing what it's meat to test.
This is what a portion of the test binary looks like:
0000000000400180 <_start>:
400180: 90 nop
0000000000400181 <main>:
400181: 90 nop
0000000000400182 <test_rip_vex2>:
400182: c5 fb 10 05 0e 00 00 vmovsd 0xe(%rip),%xmm0 # 400198 <ro_var>
400189: 00
000000000040018a <test_rip_vex2_end>:
40018a: 90 nop
The instruction at 0x400182 is the one we want to test a displaced step
for. A breakpoint is placed at 0x400182 and ran to. The execution is
then resumed from there, forcing a step-over (which should normally be a
displaced step) of the breakpoint.
However, the displaced stepping buffer is at the _start label, and that
means a breakpoint is present in the displaced stepping buffer. The
breakpoint_in_range_p check in displaced_step_prepare_throw evaluates to
true, which makes displaced_step_prepare_throw fail, forcing GDB to fall
back on an in-line step.
This can be easily observed by placing a `gdb_assert (false)` inside the
breakpoint_in_range_p condition, in displaced_step_prepare_throw, and
running gdb.arch/amd64-disp-step-avx.exp. The assertion will make the
test fail.
The proposed fix is to pad `_start` with a bunch of nops so that the
test instruction is out of the displaced step buffer.
I also think it would be good to enhance the test to make sure that we
are testing displaced stepping as intended. I did that by enabling "set
debug displaced on" while we step over the interesting instruction, and
matching a message printed only when a displaced step is executed.
gdb/testsuite/ChangeLog:
* gdb.arch/amd64-disp-step-avx.S: Add nops after _start.
* gdb.arch/amd64-disp-step-avx.exp: Enable "set debug displaced
on" while stepping over the test instruction, match printed
message.
---
gdb/testsuite/gdb.arch/amd64-disp-step-avx.S | 5 +++++
gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp | 8 +++++++-
2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/gdb/testsuite/gdb.arch/amd64-disp-step-avx.S b/gdb/testsuite/gdb.arch/amd64-disp-step-avx.S
index 76747360b9..c72f6a5ca8 100644
--- a/gdb/testsuite/gdb.arch/amd64-disp-step-avx.S
+++ b/gdb/testsuite/gdb.arch/amd64-disp-step-avx.S
@@ -22,7 +22,12 @@
.global _start,main
_start:
+ # The area at _start is used as the displaced stepping buffer. Put
+ # more than enough nop instructions so that the instructions under test
+ # below don't conflict with it.
+ .rept 200
nop
+ .endr
main:
nop
diff --git a/gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp b/gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp
index 23282f6678..3144a147f3 100644
--- a/gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp
+++ b/gdb/testsuite/gdb.arch/amd64-disp-step-avx.exp
@@ -92,10 +92,16 @@ proc disp_step_func { func } {
set value "0xdeadbeefd3adb33f"
set_regs $value
+ # Turn "debug displaced" on to make sure displaced step is actually
+ # executed, not an inline step.
+ gdb_test_no_output "set debug displaced on"
+
gdb_test "continue" \
- "Continuing.*Breakpoint.*, ${test_end_label} ().*" \
+ "Continuing.*displaced: displaced pc to.*Breakpoint.*, ${test_end_label} ().*" \
"continue to ${test_end_label}"
+ gdb_test_no_output "set debug displaced off"
+
verify_regs $value
}
--
2.25.1