From 3d4352200e3e98a6d8855e6f3a39b6d33d84e36b Mon Sep 17 00:00:00 2001 From: Tom de Vries Date: Tue, 24 Sep 2019 23:38:49 +0200 Subject: [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 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 PR gdb/24598 * gdb.arch/amd64-init-x87-values.exp: Add xfail. --- gdb/amd64-linux-nat.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'gdb/amd64-linux-nat.c') diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c index 4f1c98a..d0328b6 100644 --- a/gdb/amd64-linux-nat.c +++ b/gdb/amd64-linux-nat.c @@ -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, -- cgit v1.1