[PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[John Baldwin]

- Use signal frame sniffers that look for the signal trampoline
  instruction sequence to detect most signal frames.
- FreeBSD kernels between 9.2 and 10.1 inclusive do not include
  the signal trampoline code in process core dumps.  To detect
  signal frames for core dumps under these kernels, use the
  kern.proc.sigtramp.<pid> sysctl to fetch the location of the
  signal trampoline in the gdb process and assume that PC values
  within this location are signal frames.  This depends on that
  location being identical for all binaries.
---
 gdb/ChangeLog        |  12 ++++
 gdb/amd64fbsd-nat.c  |  34 +++++++-----
 gdb/amd64fbsd-tdep.c |  32 ++++++++++-
 gdb/i386fbsd-nat.c   |  32 ++++++-----
 gdb/i386fbsd-tdep.c  | 154 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 233 insertions(+), 31 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index af5da9b..3e1de17 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,15 @@
+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".
+
 2015-01-29  Doug Evans  <dje@google.com>
 
 	PR symtab/17890
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 abb0cab..b6467aa 100644
--- a/gdb/amd64fbsd-tdep.c
+++ b/gdb/amd64fbsd-tdep.c
@@ -31,6 +31,33 @@
 
 /* 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.  */
 
@@ -88,8 +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[] =
@@ -199,6 +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;
-- 
2.2.1

Re: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[Pedro Alves]

Thanks, fixed a few whitespace/formatting issues (tabs vs spaces,
double space after period, no () when referring to functions),
adding missing entries to ChangeLog, and pushed, as below.

Thanks again for doing this.

-------
From cf424aef0af89903abdd6c4e055684929e4990af Mon Sep 17 00:00:00 2001
From: John Baldwin <jhb@freebsd.org>
Date: Wed, 25 Feb 2015 09:51:42 -0500
Subject: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86

- Use signal frame sniffers that look for the signal trampoline
  instruction sequence to detect most signal frames.

- FreeBSD kernels between 9.2 and 10.1 inclusive do not include the
  signal trampoline code in process core dumps.  To detect signal
  frames for core dumps under these kernels, use the
  kern.proc.sigtramp.<pid> sysctl to fetch the location of the signal
  trampoline in the gdb process and assume that PC values within this
  location are signal frames.  This depends on that location being
  identical for all binaries.

gdb/ChangeLog:
2015-02-25  John Baldwin  <jhb@FreeBSD.org>

	* amd64fbsd-nat.c: Include sys/user.h.
	(_initialize_amd64fbsd_nat): Use the KERN_PROC_SIGTRAMP sysctl
	instead of KERN_PS_STRINGS to locate the signal trampoline.
	* i386fbsd-nat.c: Include sys/user.h.
	(_initialize_i386fbsd_nat): Use the KERN_PROC_SIGTRAMP sysctl
	instead of KERN_PS_STRINGS to locate the signal trampoline.
	* amd64fbsd-tdep.c (amd64fbsd_sigtramp_code): New.
	(amd64fbsd_sigtramp_p): New.
	(amd64fbsd_sigtramp_start_addr, amd64fbsd_sigtramp_end_addr): No
	longer set default values.
	(amd64fbsd_init_abi): Set "sigtramp_p" to "amd64fbsd_sigtramp_p".
	* i386fbsd-tdep.c (i386fbsd_sigtramp_start)
	(i386fbsd_sigtramp_middle, i386fbsd_sigtramp_end)
	(i386fbsd_freebsd4_sigtramp_start)
	(i386fbsd_freebsd4_sigtramp_middle)
	(i386fbsd_freebsd4_sigtramp_end, i386fbsd_osigtramp_start)
	(i386fbsd_osigtramp_middle, i386fbsd_osigtramp_end): New.
	(i386fbsd_sigtramp_p): New.
	(i386fbsd_sigtramp_start_addr, i386fbsd_sigtramp_end_addr): No
	longer set default values.
	(i386fbsd_init_abi): Set "sigtramp_p" to "i386fbsd_sigtramp_p".
---
 gdb/ChangeLog        |  24 ++++++++
 gdb/amd64fbsd-nat.c  |  34 +++++++-----
 gdb/amd64fbsd-tdep.c |  32 ++++++++++-
 gdb/i386fbsd-nat.c   |  32 ++++++-----
 gdb/i386fbsd-tdep.c  | 154 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 245 insertions(+), 31 deletions(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index b024d23..9934346 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,27 @@
+2015-02-25  John Baldwin  <jhb@FreeBSD.org>
+
+	* amd64fbsd-nat.c: Include sys/user.h.
+	(_initialize_amd64fbsd_nat): Use the KERN_PROC_SIGTRAMP sysctl
+	instead of KERN_PS_STRINGS to locate the signal trampoline.
+	* i386fbsd-nat.c: Include sys/user.h.
+	(_initialize_i386fbsd_nat): Use the KERN_PROC_SIGTRAMP sysctl
+	instead of KERN_PS_STRINGS to locate the signal trampoline.
+	* amd64fbsd-tdep.c (amd64fbsd_sigtramp_code): New.
+	(amd64fbsd_sigtramp_p): New.
+	(amd64fbsd_sigtramp_start_addr, amd64fbsd_sigtramp_end_addr): No
+	longer set default values.
+	(amd64fbsd_init_abi): Set "sigtramp_p" to "amd64fbsd_sigtramp_p".
+	* i386fbsd-tdep.c (i386fbsd_sigtramp_start)
+	(i386fbsd_sigtramp_middle, i386fbsd_sigtramp_end)
+	(i386fbsd_freebsd4_sigtramp_start)
+	(i386fbsd_freebsd4_sigtramp_middle)
+	(i386fbsd_freebsd4_sigtramp_end, i386fbsd_osigtramp_start)
+	(i386fbsd_osigtramp_middle, i386fbsd_osigtramp_end): New.
+	(i386fbsd_sigtramp_p): New.
+	(i386fbsd_sigtramp_start_addr, i386fbsd_sigtramp_end_addr): No
+	longer set default values.
+	(i386fbsd_init_abi): Set "sigtramp_p" to "i386fbsd_sigtramp_p".
+
 2015-02-25  John Baldwin  <jhb@freebsd.org>
 
 	* amd64fbsd-tdep.c (amd64fbsd_sigcontext_addr): Use
diff --git a/gdb/amd64fbsd-nat.c b/gdb/amd64fbsd-nat.c
index 1c396e2..b1b261c 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 abb0cab..e11b0f3 100644
--- a/gdb/amd64fbsd-tdep.c
+++ b/gdb/amd64fbsd-tdep.c
@@ -31,6 +31,33 @@
 
 /* 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.  */
 
@@ -88,8 +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[] =
@@ -199,6 +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..ad439e3 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..d4516ee 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;
-- 
1.9.3

Re: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[John Baldwin]

On Thursday, February 26, 2015 11:22:56 AM Pedro Alves wrote:
> Thanks, fixed a few whitespace/formatting issues (tabs vs spaces,
> double space after period, no () when referring to functions),
> adding missing entries to ChangeLog, and pushed, as below.
> 
> Thanks again for doing this.

Thanks!  I'll aim to do better about the formatting nits in future patches.

-- 
John Baldwin

Re: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[Pedro Alves]

On 02/26/2015 09:35 PM, John Baldwin wrote:
> On Thursday, February 26, 2015 11:22:56 AM Pedro Alves wrote:
>> Thanks, fixed a few whitespace/formatting issues (tabs vs spaces,
>> double space after period, no () when referring to functions),
>> adding missing entries to ChangeLog, and pushed, as below.
>>
>> Thanks again for doing this.
> 
> Thanks!  I'll aim to do better about the formatting nits in future patches.

No worries, not a bit deal.  :-)

So I saw that you're sending in more patches.  Awesome!

Would you like git commit access so you can push patches yourself?

Thanks,
Pedro Alves

Re: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[John Baldwin]

On Friday, February 27, 2015 09:46:03 AM Pedro Alves wrote:
> On 02/26/2015 09:35 PM, John Baldwin wrote:
> > On Thursday, February 26, 2015 11:22:56 AM Pedro Alves wrote:
> >> Thanks, fixed a few whitespace/formatting issues (tabs vs spaces,
> >> double space after period, no () when referring to functions),
> >> adding missing entries to ChangeLog, and pushed, as below.
> >> 
> >> Thanks again for doing this.
> > 
> > Thanks!  I'll aim to do better about the formatting nits in future
> > patches.
> 
> No worries, not a bit deal.  :-)
> 
> So I saw that you're sending in more patches.  Awesome!
> 
> Would you like git commit access so you can push patches yourself?

If you think that makes the most sense, sure.  I don't have a ton of pending
patches right now and some of the work I've done isn't quite suitable for 
upstreaming.  My current things I'd like to do are:

- the 'gcore' patch
- XSAVE/AVX support for FreeBSD/x86
- possible followup to the XSAVE patch to create a x86fbsd-nat.c similar
  to x86-linux-nat.c (need to see if the dbreg stuff in *bsd-nat.c are
  actually FreeBSD specific or if other BSD's do have the relevant
  ptrace ops

The other big thing I would eventually like to upstream is a new thread
target for FreeBSD that uses libthread_db and supports the thread libraries
used in modern FreeBSD versions.  Such a target exists now but has been
authored by several folks (and I'm not sure I can get them all to file FSF 
paperwork), so most likely this will require me to write a new thread target 
from scratch (and that would probably take me a while).

-- 
John Baldwin

Re: [PATCH 2/2] Rework signal frame probing for FreeBSD/x86.

[Pedro Alves]

Hi John,

On 02/27/2015 07:01 PM, John Baldwin wrote:
> On Friday, February 27, 2015 09:46:03 AM Pedro Alves wrote:

>> Would you like git commit access so you can push patches yourself?
> 
> If you think that makes the most sense, sure.  

I think it does.  Assuming you don't have a sourceware account yet,
please fill this form here, and list me as sponsor:

  https://sourceware.org/cgi-bin/pdw/ps_form.cgi

Thanks,
Pedro Alves