Change the way the lcall $7,$0 is reflected to usermode. Instead of

setting call gate, which must be 64 bit, put a code segment descriptor
into ldt slot 0.

This way, syscall shim does not switch temporary to 64bit trampoline,
and does not create a window where signal delivery interrupts 64 bit
mode (signal handler cannot return).  The cost is shim running with
non-zero based segment in %cs, which requires vfork() handling make
more assumptions.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

1 files changed, 14 insertions, 25 deletions

diff --git a/sys/amd64/ia32/ia32_syscall.c b/sys/amd64/ia32/ia32_syscall.c
index 92249f9..6e96edd 100644
--- a/sys/amd64/ia32/ia32_syscall.c
+++ b/sys/amd64/ia32/ia32_syscall.c
@@ -223,39 +223,28 @@ int
 setup_lcall_gate(void)
 {
 	struct i386_ldt_args uap;
-	struct user_segment_descriptor descs[2];
-	struct gate_descriptor *ssd;
+	struct user_segment_descriptor desc;
 	uint32_t lcall_addr;
 	int error;
 
 	bzero(&uap, sizeof(uap));
 	uap.start = 0;
-	uap.num = 2;
-
-	/*
-	 * This is the easiest way to cut the space for system
-	 * descriptor in ldt.  Manually adjust the descriptor type to
-	 * the call gate later.
-	 */
-	bzero(&descs[0], sizeof(descs));
-	descs[0].sd_type = SDT_SYSNULL;
-	descs[1].sd_type = SDT_SYSNULL;
-	error = amd64_set_ldt(curthread, &uap, descs);
+	uap.num = 1;
+	lcall_addr = curproc->p_sysent->sv_psstrings - sz_lcall_tramp;
+	bzero(&desc, sizeof(desc));
+	desc.sd_type = SDT_MEMERA;
+	desc.sd_dpl = SEL_UPL;
+	desc.sd_p = 1;
+	desc.sd_def32 = 1;
+	desc.sd_gran = 1;
+	desc.sd_lolimit = 0xffff;
+	desc.sd_hilimit = 0xf;
+	desc.sd_lobase = lcall_addr;
+	desc.sd_hibase = lcall_addr >> 24;
+	error = amd64_set_ldt(curthread, &uap, &desc);
 	if (error != 0)
 		return (error);
 
-	lcall_addr = curproc->p_sysent->sv_psstrings - sz_lcall_tramp;
-	mtx_lock(&dt_lock);
-	ssd = (struct gate_descriptor *)(curproc->p_md.md_ldt->ldt_base);
-	bzero(ssd, sizeof(*ssd));
-	ssd->gd_looffset = lcall_addr;
-	ssd->gd_hioffset = lcall_addr >> 16;
-	ssd->gd_selector = _ucodesel;
-	ssd->gd_type = SDT_SYSCGT;
-	ssd->gd_dpl = SEL_UPL;
-	ssd->gd_p = 1;
-	mtx_unlock(&dt_lock);
-
 	return (0);
 }
 #endif