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

John Baldwin
2015-02-16T18:25:27Z

On Wednesday, February 11, 2015 04:40:17 PM Pedro Alves wrote:
> On 02/11/2015 03:32 PM, John Baldwin wrote:
> > Actually, this does sound far simpler. I was simply updating the sigtramp
> > code that was already present. I can certainly work on changing both i386
> > and amd64 to do this instead if that is the preferred method (and it seems
> > to be from looking at other targets).
>
> Yep, that's the preferred method. That'd be great.

I've implemented this and attached the updated patch below. I'm not quite
sure if the updated Changelog is correct however. I ran into one hiccup
though which is that the signal trampoline code is not included in process
core dumps in recent FreeBSD versions (after it was moved off of the stack and
into a global shared page). I've fixed this in FreeBSD so that future
versions will include the trampoline in core dumps, but I've retained the
change to use KERN_PROC_SIGTRAMP to support core dumps from the versions that
do not include it in the core. I've removed the support for specifying a
signal trampoline location for older verions using either hardcoded offsets or
ps_strings as it is no longer needed.

ChangeLog:

2015-xx-xx John Baldwin <jhb@FreeBSD.org>

* amd64fbsd-nat.c: Include sys/user.h.
(_initialize_amd64fbsd_nat): Use the KERN_PROC_SIGTRAMP sysctl to
locate the signal trampoline.
* i386fbsd-nat.c (_initialize_i386fbsd_nat): Likewise.
* amd64fbsd-tdep.c (amd64fbsd_sigcontext_addr): Use get_frame_register.
(amd64fbsd_sigtramp_p): New.
(amd64fbsd_init_abi): Set "sigtramp_p" to "amd64fbsd_sigtramp_p".
* i386fbsd-tdep.c (i386fbsd_sigtramp_p): New.
(i386fbsd_init_abi): Set "sigtramp_p" to "i386fbsd_sigtramp_p".

Diff:

diff --git a/gdb/amd64fbsd-nat.c b/gdb/amd64fbsd-nat.c
index 1c396e2..30c8e1e 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>
#include <machine/reg.h>

#include "fbsd-nat.h"
@@ -244,24 +245,31 @@ Please report this to <bug-gdb@gnu.org>."),

SC_RBP_OFFSET = offset;

- /* 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
- still based on the assumption that the sigtramp is placed
- directly under the location where the program arguments and
- environment can be found. */
+#ifdef KERN_PROC_SIGTRAMP
+ /* Normally signal frames are detected via amd64fbsd_sigtramp_p ().
+ However, FreeBSD 9.2 through 10.1 do not include the page holding
+ the signal code in core dumps. These releases do provide a
+ kern.proc.sigtramp.<pid> sysctl that returns the location of the
+ signal trampoline for a running process. We fetch the location of
+ the current (gdb) process and use this to identify signal frames
+ in core dumps from these releases. Note that this only works for
+ core dumps of 64-bit (FreeBSD/amd64) processes and does not handle
+ core dumps of 32-bit (FreeBSD/i386) processes. */
{
- int mib[2];
- long ps_strings;
+ int mib[4];
+ struct kinfo_sigtramp kst;
size_t len;

mib[0] = CTL_KERN;
- mib[1] = KERN_PS_STRINGS;
- len = sizeof (ps_strings);
- if (sysctl (mib, 2, &ps_strings, &len, NULL, 0) == 0)
+ mib[1] = KERN_PROC;
+ mib[2] = KERN_PROC_SIGTRAMP;
+ mib[3] = getpid ();
+ len = sizeof (kst);
+ if (sysctl (mib, 4, &kst, &len, NULL, 0) == 0)
{
- amd64fbsd_sigtramp_start_addr = ps_strings - 32;
- amd64fbsd_sigtramp_end_addr = ps_strings;
+ amd64fbsd_sigtramp_start_addr = (uintptr_t) kst.ksigtramp_start;
+ amd64fbsd_sigtramp_end_addr = (uintptr_t) kst.ksigtramp_end;
}
}
+#endif
}
diff --git a/gdb/amd64fbsd-tdep.c b/gdb/amd64fbsd-tdep.c
index 2d49cdf..b6467aa 100644
--- a/gdb/amd64fbsd-tdep.c
+++ b/gdb/amd64fbsd-tdep.c
@@ -31,18 +31,49 @@

/* Support for signal handlers. */

+/* Return whether THIS_FRAME corresponds to a FreeBSD sigtramp
+ routine. */
+
+static const gdb_byte amd64fbsd_sigtramp_code[] =
+{
+ 0x48, 0x8d, 0x7c, 0x24, 0x10, /* lea SIGF_UC(%rsp),%rdi */
+ 0x6a, 0x00, /* pushq $0 */
+ 0x48, 0xc7, 0xc0, 0xa1, 0x01, 0x00, 0x00,
+ /* movq $SYS_sigreturn,%rax */
+ 0x0f, 0x05 /* syscall */
+};
+
+static int
+amd64fbsd_sigtramp_p (struct frame_info *this_frame)
+{
+ CORE_ADDR pc = get_frame_pc (this_frame);
+ gdb_byte buf[sizeof amd64fbsd_sigtramp_code];
+
+ if (!safe_frame_unwind_memory (this_frame, pc, buf, sizeof buf))
+ return 0;
+ if (memcmp (buf, amd64fbsd_sigtramp_code, sizeof amd64fbsd_sigtramp_code) !=
+ 0)
+ return 0;
+
+ return 1;
+}
+
/* Assuming THIS_FRAME is for a BSD sigtramp routine, return the
address of the associated sigcontext structure. */

static CORE_ADDR
amd64fbsd_sigcontext_addr (struct frame_info *this_frame)
{
+ struct gdbarch *gdbarch = get_frame_arch (this_frame);
+ enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
CORE_ADDR sp;
+ gdb_byte buf[8];

/* The `struct sigcontext' (which really is an `ucontext_t' on
FreeBSD/amd64) lives at a fixed offset in the signal frame. See
<machine/sigframe.h>. */
- sp = frame_unwind_register_unsigned (this_frame, AMD64_RSP_REGNUM);
+ get_frame_register (this_frame, AMD64_RSP_REGNUM, buf);
+ sp = extract_unsigned_integer (buf, 8, byte_order);
return sp + 16;
}
\f
@@ -84,8 +115,8 @@ static int amd64fbsd_r_reg_offset[] =
};

/* Location of the signal trampoline. */
-CORE_ADDR amd64fbsd_sigtramp_start_addr = 0x7fffffffffc0ULL;
-CORE_ADDR amd64fbsd_sigtramp_end_addr = 0x7fffffffffe0ULL;
+CORE_ADDR amd64fbsd_sigtramp_start_addr;
+CORE_ADDR amd64fbsd_sigtramp_end_addr;

/* From <machine/signal.h>. */
int amd64fbsd_sc_reg_offset[] =
@@ -195,6 +226,7 @@ amd64fbsd_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)

amd64_init_abi (info, gdbarch);

+ tdep->sigtramp_p = amd64fbsd_sigtramp_p;
tdep->sigtramp_start = amd64fbsd_sigtramp_start_addr;
tdep->sigtramp_end = amd64fbsd_sigtramp_end_addr;
tdep->sigcontext_addr = amd64fbsd_sigcontext_addr;
diff --git a/gdb/i386fbsd-nat.c b/gdb/i386fbsd-nat.c
index f4951d1..1d5e4fd 100644
--- a/gdb/i386fbsd-nat.c
+++ b/gdb/i386fbsd-nat.c
@@ -25,6 +25,7 @@
#include <sys/types.h>
#include <sys/ptrace.h>
#include <sys/sysctl.h>
+#include <sys/user.h>

#include "fbsd-nat.h"
#include "i386-tdep.h"
@@ -148,25 +149,28 @@ _initialize_i386fbsd_nat (void)
/* Support debugging kernel virtual memory images. */
bsd_kvm_add_target (i386fbsd_supply_pcb);

- /* 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
- still based on the assumption that the sigtramp is placed
- directly under the location where the program arguments and
- environment can be found. */
-#ifdef KERN_PS_STRINGS
+#ifdef KERN_PROC_SIGTRAMP
+ /* Normally signal frames are detected via i386fbsd_sigtramp_p ().
+ However, FreeBSD 9.2 through 10.1 do not include the page holding
+ the signal code in core dumps. These releases do provide a
+ kern.proc.sigtramp.<pid> sysctl that returns the location of the
+ signal trampoline for a running process. We fetch the location of
+ the current (gdb) process and use this to identify signal frames
+ in core dumps from these releases. */
{
- int mib[2];
- u_long ps_strings;
+ int mib[4];
+ struct kinfo_sigtramp kst;
size_t len;

mib[0] = CTL_KERN;
- mib[1] = KERN_PS_STRINGS;
- len = sizeof (ps_strings);
- if (sysctl (mib, 2, &ps_strings, &len, NULL, 0) == 0)
+ mib[1] = KERN_PROC;
+ mib[2] = KERN_PROC_SIGTRAMP;
+ mib[3] = getpid ();
+ len = sizeof (kst);
+ if (sysctl (mib, 4, &kst, &len, NULL, 0) == 0)
{
- i386fbsd_sigtramp_start_addr = ps_strings - 128;
- i386fbsd_sigtramp_end_addr = ps_strings;
+ i386fbsd_sigtramp_start_addr = (uintptr_t) kst.ksigtramp_start;
+ i386fbsd_sigtramp_end_addr = (uintptr_t) kst.ksigtramp_end;
}
}
#endif
diff --git a/gdb/i386fbsd-tdep.c b/gdb/i386fbsd-tdep.c
index 8d237f0..113c7c5 100644
--- a/gdb/i386fbsd-tdep.c
+++ b/gdb/i386fbsd-tdep.c
@@ -29,6 +29,154 @@
#include "fbsd-tdep.h"
#include "solib-svr4.h"

+/* Support for signal handlers. */
+
+/* Return whether THIS_FRAME corresponds to a FreeBSD sigtramp
+ routine. */
+
+/* FreeBSD/i386 supports three different signal trampolines, one for
+ versions before 4.0, a second for 4.x, and a third for 5.0 and
+ later. To complicate matters, FreeBSD/i386 binaries running under
+ an amd64 kernel use a different set of trampolines. These
+ trampolines differ from the i386 kernel trampolines in that they
+ omit a middle section that conditionally restores %gs. */
+
+static const gdb_byte i386fbsd_sigtramp_start[] =
+{
+ 0x8d, 0x44, 0x24, 0x20, /* lea SIGF_UC(%esp),%eax */
+ 0x50 /* pushl %eax */
+};
+
+static const gdb_byte i386fbsd_sigtramp_middle[] =
+{
+ 0xf7, 0x40, 0x54, 0x00, 0x00, 0x02, 0x00,
+ /* testl $PSL_VM,UC_EFLAGS(%eax) */
+ 0x75, 0x03, /* jne +3 */
+ 0x8e, 0x68, 0x14 /* mov UC_GS(%eax),%gs */
+};
+
+static const gdb_byte i386fbsd_sigtramp_end[] =
+{
+ 0xb8, 0xa1, 0x01, 0x00, 0x00, /* movl $SYS_sigreturn,%eax */
+ 0x50, /* pushl %eax */
+ 0xcd, 0x80 /* int $0x80 */
+};
+
+static const gdb_byte i386fbsd_freebsd4_sigtramp_start[] =
+{
+ 0x8d, 0x44, 0x24, 0x14, /* lea SIGF_UC4(%esp),%eax */
+ 0x50 /* pushl %eax */
+};
+
+static const gdb_byte i386fbsd_freebsd4_sigtramp_middle[] =
+{
+ 0xf7, 0x40, 0x54, 0x00, 0x00, 0x02, 0x00,
+ /* testl $PSL_VM,UC4_EFLAGS(%eax) */
+ 0x75, 0x03, /* jne +3 */
+ 0x8e, 0x68, 0x14 /* mov UC4_GS(%eax),%gs */
+};
+
+static const gdb_byte i386fbsd_freebsd4_sigtramp_end[] =
+{
+ 0xb8, 0x58, 0x01, 0x00, 0x00, /* movl $344,%eax */
+ 0x50, /* pushl %eax */
+ 0xcd, 0x80 /* int $0x80 */
+};
+
+static const gdb_byte i386fbsd_osigtramp_start[] =
+{
+ 0x8d, 0x44, 0x24, 0x14, /* lea SIGF_SC(%esp),%eax */
+ 0x50 /* pushl %eax */
+};
+
+static const gdb_byte i386fbsd_osigtramp_middle[] =
+{
+ 0xf7, 0x40, 0x18, 0x00, 0x00, 0x02, 0x00,
+ /* testl $PSL_VM,SC_PS(%eax) */
+ 0x75, 0x03, /* jne +3 */
+ 0x8e, 0x68, 0x44 /* mov SC_GS(%eax),%gs */
+};
+
+static const gdb_byte i386fbsd_osigtramp_end[] =
+{
+ 0xb8, 0x67, 0x00, 0x00, 0x00, /* movl $103,%eax */
+ 0x50, /* pushl %eax */
+ 0xcd, 0x80 /* int $0x80 */
+};
+
+/* The three different trampolines are all the same size. */
+gdb_static_assert (sizeof i386fbsd_sigtramp_start ==
+ sizeof i386fbsd_freebsd4_sigtramp_start);
+gdb_static_assert (sizeof i386fbsd_sigtramp_start ==
+ sizeof i386fbsd_osigtramp_start);
+gdb_static_assert (sizeof i386fbsd_sigtramp_middle ==
+ sizeof i386fbsd_freebsd4_sigtramp_middle);
+gdb_static_assert (sizeof i386fbsd_sigtramp_middle ==
+ sizeof i386fbsd_osigtramp_middle);
+gdb_static_assert (sizeof i386fbsd_sigtramp_end ==
+ sizeof i386fbsd_freebsd4_sigtramp_end);
+gdb_static_assert (sizeof i386fbsd_sigtramp_end ==
+ sizeof i386fbsd_osigtramp_end);
+
+/* We assume that the middle is the largest chunk below. */
+gdb_static_assert (sizeof i386fbsd_sigtramp_middle >
+ sizeof i386fbsd_sigtramp_start);
+gdb_static_assert (sizeof i386fbsd_sigtramp_middle >
+ sizeof i386fbsd_sigtramp_end);
+
+static int
+i386fbsd_sigtramp_p (struct frame_info *this_frame)
+{
+ CORE_ADDR pc = get_frame_pc (this_frame);
+ gdb_byte buf[sizeof i386fbsd_sigtramp_middle];
+ const gdb_byte *middle, *end;
+
+ /* Look for a matching start. */
+ if (!safe_frame_unwind_memory (this_frame, pc, buf,
+ sizeof i386fbsd_sigtramp_start))
+ return 0;
+ if (memcmp (buf, i386fbsd_sigtramp_start, sizeof i386fbsd_sigtramp_start) ==
+ 0) {
+ middle = i386fbsd_sigtramp_middle;
+ end = i386fbsd_sigtramp_end;
+ } else if (memcmp (buf, i386fbsd_freebsd4_sigtramp_start,
+ sizeof i386fbsd_freebsd4_sigtramp_start) == 0) {
+ middle = i386fbsd_freebsd4_sigtramp_middle;
+ end = i386fbsd_freebsd4_sigtramp_end;
+ } else if (memcmp (buf, i386fbsd_osigtramp_start,
+ sizeof i386fbsd_osigtramp_start) == 0) {
+ middle = i386fbsd_osigtramp_middle;
+ end = i386fbsd_osigtramp_end;
+ } else
+ return 0;
+
+ /* Since the end is shorter than the middle, check for a matching end
+ next. */
+ pc += sizeof i386fbsd_sigtramp_start;
+ if (!safe_frame_unwind_memory (this_frame, pc, buf,
+ sizeof i386fbsd_sigtramp_end))
+ return 0;
+ if (memcmp (buf, end, sizeof i386fbsd_sigtramp_end) == 0)
+ return 1;
+
+ /* If the end didn't match, check for a matching middle. */
+ if (!safe_frame_unwind_memory (this_frame, pc, buf,
+ sizeof i386fbsd_sigtramp_middle))
+ return 0;
+ if (memcmp (buf, middle, sizeof i386fbsd_sigtramp_middle) != 0)
+ return 0;
+
+ /* The middle matched, check for a matching end. */
+ pc += sizeof i386fbsd_sigtramp_middle;
+ if (!safe_frame_unwind_memory (this_frame, pc, buf,
+ sizeof i386fbsd_sigtramp_end))
+ return 0;
+ if (memcmp (buf, end, sizeof i386fbsd_sigtramp_end) != 0)
+ return 0;
+
+ return 1;
+}
+
/* FreeBSD 3.0-RELEASE or later. */

/* From <machine/reg.h>. */
@@ -43,8 +191,8 @@ static int i386fbsd_r_reg_offset[] =
};

/* Sigtramp routine location. */
-CORE_ADDR i386fbsd_sigtramp_start_addr = 0xbfbfdf20;
-CORE_ADDR i386fbsd_sigtramp_end_addr = 0xbfbfdff0;
+CORE_ADDR i386fbsd_sigtramp_start_addr;
+CORE_ADDR i386fbsd_sigtramp_end_addr;

/* From <machine/signal.h>. */
int i386fbsd_sc_reg_offset[] =
@@ -139,6 +287,8 @@ i386fbsdaout_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
/* FreeBSD uses -freg-struct-return by default. */
tdep->struct_return = reg_struct_return;

+ tdep->sigtramp_p = i386fbsd_sigtramp_p;
+
/* FreeBSD uses a different memory layout. */
tdep->sigtramp_start = i386fbsd_sigtramp_start_addr;
tdep->sigtramp_end = i386fbsd_sigtramp_end_addr;

--
John Baldwin