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[gdb/tdep] Handle mxcsr kernel bug on Intel Skylake CPUs 

On my openSUSE Leap 15.1 x86_64 Skylake system with the default (4.12) kernel,
I run into:
...
FAIL: gdb.base/gcore.exp: corefile restored all registers
...

The problem is that there's a difference in the mxcsr register value before
and after the gcore command:
...
- mxcsr          0x0                 [ ]
+ mxcsr          0x400440            [ DAZ OM ]
...

This can be traced back to amd64_linux_nat_target::fetch_registers, where
xstateregs is partially initialized by the ptrace call:
...
          char xstateregs[X86_XSTATE_MAX_SIZE];
          struct iovec iov;

          amd64_collect_xsave (regcache, -1, xstateregs, 0);
          iov.iov_base = xstateregs;
          iov.iov_len = sizeof (xstateregs);
          if (ptrace (PTRACE_GETREGSET, tid,
                      (unsigned int) NT_X86_XSTATE, (long) &iov) < 0)
            perror_with_name (_("Couldn't get extended state status"));

          amd64_supply_xsave (regcache, -1, xstateregs);
...
after which amd64_supply_xsave is called.

The amd64_supply_xsave call is supposed to only use initialized parts of
xstateregs, but due to a kernel bug on intel skylake (fixed from 4.14 onwards
by commit 0852b374173b "x86/fpu: Add FPU state copying quirk to handle XRSTOR
failure on Intel Skylake CPUs") it can happen that the mxcsr part of
xstateregs is not initialized, while amd64_supply_xsave expects it to be
initialized, which explains the FAIL mentioned above.

Fix the undetermined behaviour by initializing xstateregs before calling
ptrace, which makes sure we get a 0x0 for mxcsr when this kernel bug occurs,
and which also happens to fix the FAIL.

Furthermore, add an xfail for this FAIL which triggers the same kernel bug:
...
FAIL: gdb.arch/amd64-init-x87-values.exp: check_setting_mxcsr_before_enable: \
  check new value of MXCSR is still in place
...

Both FAILs pass when using a 5.3 kernel instead on the system mentioned above.

Tested on x86_64-linux.

gdb/ChangeLog:

2019-09-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/23815
	* amd64-linux-nat.c (amd64_linux_nat_target::fetch_registers):
	Initialize xstateregs before ptrace PTRACE_GETREGSET call.

gdb/testsuite/ChangeLog:

2019-09-24  Tom de Vries  <tdevries@suse.de>

	PR gdb/24598
	* gdb.arch/amd64-init-x87-values.exp: Add xfail.
This commit is contained in:
Tom de Vries 2019-09-24 23:38:49 +02:00
parent e8f8842d90
commit 3d4352200e
4 changed files with 34 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
gdb/ChangeLog
View File

@ -1,3 +1,9 @@
2019-09-24 Tom de Vries <tdevries@suse.de>
PR gdb/23815
* amd64-linux-nat.c (amd64_linux_nat_target::fetch_registers):
Initialize xstateregs before ptrace PTRACE_GETREGSET call.
2019-09-23 Dimitar Dimitrov <dimitar@dinux.eu>
* NEWS: Mention new simulator port for PRU.

6
gdb/amd64-linux-nat.c
View File

@ -238,6 +238,12 @@ amd64_linux_nat_target::fetch_registers (struct regcache *regcache, int regnum)
char xstateregs[X86_XSTATE_MAX_SIZE];
struct iovec iov;
/* Pre-4.14 kernels have a bug (fixed by commit 0852b374173b
"x86/fpu: Add FPU state copying quirk to handle XRSTOR failure on
Intel Skylake CPUs") that sometimes causes the mxcsr location in
xstateregs not to be copied by PTRACE_GETREGSET. Make sure that
the location is at least initialized with a defined value. */
memset (xstateregs, 0, sizeof (xstateregs));
iov.iov_base = xstateregs;
iov.iov_len = sizeof (xstateregs);
if (ptrace (PTRACE_GETREGSET, tid,

5
gdb/testsuite/ChangeLog
View File

@ -1,3 +1,8 @@
2019-09-24 Tom de Vries <tdevries@suse.de>
PR gdb/24598
* gdb.arch/amd64-init-x87-values.exp: Add xfail.
2019-09-22 Tom de Vries <tdevries@suse.de>
* gdb.base/restore.exp: Allow register variables to be optimized out at

19
gdb/testsuite/gdb.arch/amd64-init-x87-values.exp
View File

@ -116,7 +116,7 @@ proc_with_prefix check_x87_regs_around_init {} {
# nop that does not enable any FP features). Finally check that the
# mxcsr register still has the value we set.
proc_with_prefix check_setting_mxcsr_before_enable {} {
global binfile
global binfile gdb_prompt
clean_restart ${binfile}
@ -127,7 +127,22 @@ proc_with_prefix check_setting_mxcsr_before_enable {} {
gdb_test_no_output "set \$mxcsr=0x9f80" "set a new value for MXCSR"
gdb_test "stepi" "fwait" "step forward one instruction for mxcsr test"
gdb_test "p/x \$mxcsr" " = 0x9f80" "check new value of MXCSR is still in place"
set test "check new value of MXCSR is still in place"
set pass_pattern " = 0x9f80"
# Pre-4.14 kernels have a bug (fixed by commit 0852b374173b "x86/fpu:
# Add FPU state copying quirk to handle XRSTOR failure on Intel Skylake
# CPUs") that causes mxcsr not to be copied, in which case we get 0 instead of
# the just saved value.
set xfail_pattern " = 0x0"
gdb_test_multiple "p/x \$mxcsr" $test {
-re "\[\r\n\]*(?:$pass_pattern)\[\r\n\]+$gdb_prompt $" {
pass $test
}
-re "\[\r\n\]*(?:$xfail_pattern)\[\r\n\]+$gdb_prompt $" {
xfail $test
}
}
}
# Start the test file, all FP features will be disabled. Set new