diff --git a/contrib/elftoolchain/readelf/readelf.c b/contrib/elftoolchain/readelf/readelf.c
--- a/contrib/elftoolchain/readelf/readelf.c
+++ b/contrib/elftoolchain/readelf/readelf.c
@@ -1195,6 +1195,7 @@
 	case 17: return "NT_PTLWPINFO";
 	case 0x100: return "NT_PPC_VMX (ppc Altivec registers)";
 	case 0x102: return "NT_PPC_VSX (ppc VSX registers)";
+	case 0x200: return "NT_X86_SEGBASES (x86 segment base registers)";
 	case 0x202: return "NT_X86_XSTATE (x86 XSAVE extended state)";
 	case 0x400: return "NT_ARM_VFP (arm VFP registers)";
 	case 0x401: return "NT_ARM_TLS (arm TLS register)";
diff --git a/sys/amd64/amd64/ptrace_machdep.c b/sys/amd64/amd64/ptrace_machdep.c
--- a/sys/amd64/amd64/ptrace_machdep.c
+++ b/sys/amd64/amd64/ptrace_machdep.c
@@ -32,11 +32,13 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/elf.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
 #include <sys/ptrace.h>
+#include <sys/reg.h>
 #include <sys/sysent.h>
 #include <vm/vm.h>
 #include <vm/pmap.h>
@@ -52,6 +54,106 @@
 };
 #endif
 
+static bool
+get_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t *sizep)
+{
+	struct segbasereg *reg;
+	struct pcb *pcb;
+
+	if (buf != NULL) {
+		KASSERT(*sizep == sizeof(*reg), ("%s: invalid size", __func__));
+		reg = buf;
+
+		pcb = td->td_pcb;
+		if (td == curthread)
+			update_pcb_bases(pcb);
+		reg->r_fsbase = pcb->pcb_fsbase;
+		reg->r_gsbase = pcb->pcb_gsbase;
+	}
+	*sizep = sizeof(*reg);
+	return (true);
+}
+
+static bool
+set_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t size)
+{
+	struct segbasereg *reg;
+	struct pcb *pcb;
+
+	KASSERT(size == sizeof(*reg), ("%s: invalid size", __func__));
+	reg = buf;
+
+	pcb = td->td_pcb;
+	set_pcb_flags(pcb, PCB_FULL_IRET);
+	pcb->pcb_fsbase = reg->r_fsbase;
+	td->td_frame->tf_fs = _ufssel;
+	pcb->pcb_gsbase = reg->r_gsbase;
+	td->td_frame->tf_gs = _ugssel;
+
+	return (true);
+}
+
+static struct regset regset_segbases = {
+	.note = NT_X86_SEGBASES,
+	.size = sizeof(struct segbasereg),
+	.get = get_segbases,
+	.set = set_segbases,
+};
+ELF_REGSET(regset_segbases);
+
+#ifdef COMPAT_FREEBSD32
+static bool
+get_segbases32(struct regset *rs, struct thread *td, void *buf,
+    size_t *sizep)
+{
+	struct segbasereg32 *reg;
+	struct pcb *pcb;
+
+	if (buf != NULL) {
+		KASSERT(*sizep == sizeof(*reg), ("%s: invalid size", __func__));
+		reg = buf;
+
+		pcb = td->td_pcb;
+		if (td == curthread)
+			update_pcb_bases(pcb);
+		reg->r_fsbase = (uint32_t)pcb->pcb_fsbase;
+		reg->r_gsbase = (uint32_t)pcb->pcb_gsbase;
+	}
+	*sizep = sizeof(*reg);
+	return (true);
+}
+
+static bool
+set_segbases32(struct regset *rs, struct thread *td, void *buf,
+    size_t size)
+{
+	struct segbasereg32 *reg;
+	struct pcb *pcb;
+
+	KASSERT(size == sizeof(*reg), ("%s: invalid size", __func__));
+	reg = buf;
+
+	pcb = td->td_pcb;
+	set_pcb_flags(pcb, PCB_FULL_IRET);
+	pcb->pcb_fsbase = reg->r_fsbase;
+	td->td_frame->tf_fs = _ufssel;
+	pcb->pcb_gsbase = reg->r_gsbase;
+	td->td_frame->tf_gs = _ugssel;
+
+	return (true);
+}
+
+static struct regset regset_segbases32 = {
+	.note = NT_X86_SEGBASES,
+	.size = sizeof(struct segbasereg32),
+	.get = get_segbases32,
+	.set = set_segbases32,
+};
+ELF32_REGSET(regset_segbases32);
+#endif
+
 static int
 cpu_ptrace_xstate(struct thread *td, int req, void *addr, int data)
 {
diff --git a/sys/amd64/ia32/ia32_reg.c b/sys/amd64/ia32/ia32_reg.c
--- a/sys/amd64/ia32/ia32_reg.c
+++ b/sys/amd64/ia32/ia32_reg.c
@@ -32,6 +32,7 @@
 __FBSDID("$FreeBSD$");
 
 #include <sys/param.h>
+#include <sys/elf.h>
 #include <sys/exec.h>
 #include <sys/fcntl.h>
 #include <sys/imgact.h>
@@ -266,3 +267,52 @@
 		dr.dr[i] = 0;
 	return (set_dbregs(td, &dr));
 }
+
+static bool
+get_i386_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t *sizep)
+{
+	struct segbasereg32 *reg;
+	struct pcb *pcb;
+
+	if (buf != NULL) {
+		KASSERT(*sizep == sizeof(*reg), ("%s: invalid size", __func__));
+		reg = buf;
+
+		pcb = td->td_pcb;
+		if (td == curthread)
+			update_pcb_bases(pcb);
+		reg->r_fsbase = pcb->pcb_fsbase;
+		reg->r_gsbase = pcb->pcb_gsbase;
+	}
+	*sizep = sizeof(*reg);
+	return (true);
+}
+
+static bool
+set_i386_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t size)
+{
+	struct segbasereg32 *reg;
+	struct pcb *pcb;
+
+	KASSERT(size == sizeof(*reg), ("%s: invalid size", __func__));
+	reg = buf;
+
+	pcb = td->td_pcb;
+	set_pcb_flags(pcb, PCB_FULL_IRET);
+	pcb->pcb_fsbase = reg->r_fsbase;
+	td->td_frame->tf_fs = _ufssel;
+	pcb->pcb_gsbase = reg->r_gsbase;
+	td->td_frame->tf_gs = _ugssel;
+
+	return (true);
+}
+
+static struct regset regset_i386_segbases = {
+	.note = NT_X86_SEGBASES,
+	.size = sizeof(struct segbasereg),
+	.get = get_i386_segbases,
+	.set = set_i386_segbases,
+};
+ELF32_REGSET(regset_i386_segbases);
diff --git a/sys/i386/i386/ptrace_machdep.c b/sys/i386/i386/ptrace_machdep.c
--- a/sys/i386/i386/ptrace_machdep.c
+++ b/sys/i386/i386/ptrace_machdep.c
@@ -34,15 +34,64 @@
 
 #include <sys/param.h>
 #include <sys/systm.h>
+#include <sys/elf.h>
 #include <sys/lock.h>
 #include <sys/malloc.h>
 #include <sys/mutex.h>
 #include <sys/proc.h>
 #include <sys/ptrace.h>
+#include <sys/reg.h>
 #include <machine/frame.h>
 #include <machine/md_var.h>
 #include <machine/pcb.h>
 
+static uint32_t
+get_segbase(struct segment_descriptor *sdp)
+{
+	return (sdp->sd_hibase << 24 | sdp->sd_lobase);
+}
+
+static bool
+get_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t *sizep)
+{
+	struct segbasereg *reg;
+
+	if (buf != NULL) {
+		KASSERT(*sizep == sizeof(*reg), ("%s: invalid size", __func__));
+		reg = buf;
+		reg->r_fsbase = get_segbase(&td->td_pcb->pcb_fsd);
+		reg->r_gsbase = get_segbase(&td->td_pcb->pcb_gsd);
+	}
+	*sizep = sizeof(*reg);
+	return (true);
+}
+
+static bool
+set_segbases(struct regset *rs, struct thread *td, void *buf,
+    size_t size)
+{
+	struct segbasereg *reg;
+
+	KASSERT(size == sizeof(*reg), ("%s: invalid size", __func__));
+	reg = buf;
+
+	fill_based_sd(&td->td_pcb->pcb_fsd, reg->r_fsbase);
+	td->td_frame->tf_fs = GSEL(GUFS_SEL, SEL_UPL);
+	fill_based_sd(&td->td_pcb->pcb_gsd, reg->r_gsbase);
+	td->td_pcb->pcb_gs = GSEL(GUGS_SEL, SEL_UPL);
+
+	return (true);
+}
+
+static struct regset regset_segbases = {
+	.note = NT_X86_SEGBASES,
+	.size = sizeof(struct segbasereg),
+	.get = get_segbases,
+	.set = set_segbases,
+};
+ELF_REGSET(regset_segbases);
+
 static int
 cpu_ptrace_xstate(struct thread *td, int req, void *addr, int data)
 {
@@ -173,7 +222,7 @@
 	case PT_GETGSBASE:
 		sdp = req == PT_GETFSBASE ? &td->td_pcb->pcb_fsd :
 		    &td->td_pcb->pcb_gsd;
-		r = sdp->sd_hibase << 24 | sdp->sd_lobase;
+		r = get_segbase(sdp);
 		error = copyout(&r, addr, sizeof(r));
 		break;
 
diff --git a/sys/sys/elf_common.h b/sys/sys/elf_common.h
--- a/sys/sys/elf_common.h
+++ b/sys/sys/elf_common.h
@@ -823,6 +823,7 @@
 #define	NT_PTLWPINFO		17	/* Thread ptrace miscellaneous info. */
 #define	NT_PPC_VMX	0x100	/* PowerPC Altivec/VMX registers */
 #define	NT_PPC_VSX	0x102	/* PowerPC VSX registers */
+#define	NT_X86_SEGBASES	0x200	/* x86 FS/GS base addresses. */
 #define	NT_X86_XSTATE	0x202	/* x86 XSAVE extended state. */
 #define	NT_ARM_VFP	0x400	/* ARM VFP registers */
 #define	NT_ARM_TLS	0x401	/* ARM TLS register */
diff --git a/sys/x86/include/reg.h b/sys/x86/include/reg.h
--- a/sys/x86/include/reg.h
+++ b/sys/x86/include/reg.h
@@ -86,6 +86,7 @@
 #define	__reg32		reg
 #define	__fpreg32	fpreg
 #define	__dbreg32	dbreg
+#define	__segbasereg32	segbasereg
 #else
 #define	__reg32		reg32
 #define	__reg64		reg
@@ -93,6 +94,8 @@
 #define	__fpreg64	fpreg
 #define	__dbreg32	dbreg32
 #define	__dbreg64	dbreg
+#define	__segbasereg32	segbasereg32
+#define	__segbasereg64	segbasereg
 #define	__HAVE_REG32
 #endif
 
@@ -236,12 +239,27 @@
 #define	DBREG_DRX(d,x)	((d)->dr[(x)])	/* reference dr0 - dr7 by
 					   register number */
 
+/*
+ * Register set accessible via NT_X86_SEGBASES.
+ */
+struct __segbasereg32 {
+	__uint32_t	r_fsbase;
+	__uint32_t	r_gsbase;
+};
+
+struct __segbasereg64 {
+	__uint64_t	r_fsbase;
+	__uint64_t	r_gsbase;
+};
+
 #undef __reg32
 #undef __reg64
 #undef __fpreg32
 #undef __fpreg64
 #undef __dbreg32
 #undef __dbreg64
+#undef __segbasereg32
+#undef __segbasereg64
 
 #ifdef _KERNEL
 /*
diff --git a/usr.bin/gcore/elfcore.c b/usr.bin/gcore/elfcore.c
--- a/usr.bin/gcore/elfcore.c
+++ b/usr.bin/gcore/elfcore.c
@@ -379,6 +379,7 @@
 		elf_putregnote(NT_ARM_VFP, tids[i], sb);
 #endif
 #if defined(__i386__) || defined(__amd64__)
+		elf_putregnote(NT_X86_SEGBASES, tids[i], sb);
 		elf_putnote(NT_X86_XSTATE, elf_note_x86_xstate, tids + i, sb);
 #endif
 #if defined(__powerpc__)