Re: [PATCH] Fix signal trampoline detection/unwinding on recent FreeBSD/i386 and FreeBSD/amd64

Pedro Alves
2015-02-10T17:08:21Z

On 02/10/2015 02:50 PM, John Baldwin wrote:
> On Wednesday, February 04, 2015 10:47:07 AM John Baldwin wrote:
>> This patch fixes two issues with signal frame unwinding on recent
>> FreeBSD/x86:
>>
>> First, FreeBSD moved the signal trampoline code into a global shared page
>> exported by the kernel a few releases ago. To try to make this easier to
>> detect, FreeBSD added a new per-process sysctl node that exports the
>> starting and ending address of the signal trampoline code. This sysctl
>> works on all platforms that FreeBSD supports no matter where the signal
>> trampoline lives, but I have only updated i386 and amd64 in this patch.
>>
>> Second, amd64fbsd_sigcontext_addr was using frame_unwind_register_unsigned
>> to fetch the stack pointer which resulted in infinite recursion. I've
>> changed it to use get_frame_register() to match the sigcontext_addr methods
>> in the i386-bsd and amd64-linux targets instead.
>
> Does anyone have any comments on this patch or is there anything I need to fix
> in it?
>
>> Changelog:
>>
>> 2015-xx-xx John Baldwin <jhb@FreeBSD.org>
>>
>> * amd64fbsd-nat.c (_initialize_amd64fbsd_nat): Use the KERN_PROC_SIGTRAMP
>> sysctl to locate the signal trampoline.

"sysctl" should be aligned to the tab as well (under the star).

>> * i386fbsd-nat.c (_initialize_i386fbsd_nat): Likewise.
>> * amd64fbsd-tdep.c (amd64fbsd_sigcontext_addr): Fix infinite recursion.

"Fix infinite recursion." is what goes in the commit log. Here say:

* amd64fbsd-tdep.c (amd64fbsd_sigcontext_addr): Use get_frame_register.

>>
>> diff --git a/gdb/amd64fbsd-nat.c b/gdb/amd64fbsd-nat.c
>> index 1c396e2..6227681 100644
>> --- a/gdb/amd64fbsd-nat.c
>> +++ b/gdb/amd64fbsd-nat.c
>> @@ -26,6 +26,7 @@
>> #include <sys/types.h>
>> #include <sys/ptrace.h>
>> #include <sys/sysctl.h>
>> +#include <sys/user.h>

Mention this new inclusion in the CL:

* amd64fbsd-nat.c: Include sys/user.h.
(_initialize_amd64fbsd_nat): Use the KERN_PROC_SIGTRAMP
sysctl to locate the signal trampoline.

>> #include <machine/reg.h>
>>
>> #include "fbsd-nat.h"
>> @@ -244,6 +245,29 @@ Please report this to <bug-gdb@gnu.org>."),
>>
>> SC_RBP_OFFSET = offset;
>>
>> +#ifdef KERN_PROC_SIGTRAMP
>> + /* Newer versions of FreeBSD provide a kern.proc.sigtramp.<pid>

"Newer" will get old soon. Can you replace that with some kind of
version number/name?

>> + sysctl that returns the location of the signal trampoline.
>> + Note that this fetches the address for the current (gdb) process.
>> + This will be correct for other 64-bit processes, but the signal
>> + trampoline location is not properly set for 32-bit processes. */

Double-space after periods: "processes. */"

I'm not sure I understand what does "but the signal trampoline
location is not properly set for 32-bit processes" means. You mean
it's not properly set because GDB is 64-bit; or it's not properly set
in the kernel; or something else?

>> + {
>> + int mib[4];
>> + struct kinfo_sigtramp kst;
>> + size_t len;
>> +
>> + mib[0] = CTL_KERN;
>> + mib[1] = KERN_PROC;
>> + mib[2] = KERN_PROC_SIGTRAMP;
>> + mib[3] = getpid();

Space before parens:

mib[3] = getpid ();

>> + len = sizeof (kst);
>> + if (sysctl (mib, 4, &kst, &len, NULL, 0) == 0)
>> + {
>> + amd64fbsd_sigtramp_start_addr = (uintptr_t)kst.ksigtramp_start;
>> + amd64fbsd_sigtramp_end_addr = (uintptr_t)kst.ksigtramp_end;

In casts too:

amd64fbsd_sigtramp_start_addr = (uintptr_t) kst.ksigtramp_start;
amd64fbsd_sigtramp_end_addr = (uintptr_t) kst.ksigtramp_end;

>> + }
>> + }
>> +#else

Did you consider making GDB fallback to the #else block at runtime, for the
case GDB that was built against newer FreeBSD headers but is actually
running on older FreeBSD ?

>> /* FreeBSD provides a kern.ps_strings sysctl that we can use to
>> locate the sigtramp. That way we can still recognize a sigtramp
>> if its location is changed in a new kernel. Of course this is
>> @@ -264,4 +288,5 @@ Please report this to <bug-gdb@gnu.org>."),
>> amd64fbsd_sigtramp_end_addr = ps_strings;
>> }
>> }
>> +#endif
>> }

Otherwise looks good to me too.

Thanks,
Pedro Alves