Fix makecontext() on MIPS O32.

The GP register can be clobbered by the callback, so save it in S1
while invoking the callback function.

While here, add a comment expounding on the treatment of GP for the
various ABIs and the assumptions made.

Reviewed by:	jmallett (earlier version)
Sponsored by:	DARPA / AFRL
Differential Revision:	https://reviews.freebsd.org/D14179

1 files changed, 15 insertions, 1 deletions

diff --git a/lib/libc/mips/gen/_ctx_start.S b/lib/libc/mips/gen/_ctx_start.S
index 5d754a9927d1..f1f5cdc32641 100644
--- a/lib/libc/mips/gen/_ctx_start.S
+++ b/lib/libc/mips/gen/_ctx_start.S
@@ -28,11 +28,25 @@
 __FBSDID("$FreeBSD$");
 
 /*
- * XXX gp?
+ * This requires makecontext() to setup a valid GP for locating
+ * _ctx_done rather than deriving GP from T9 on entry.  Currently this
+ * uses the GP inherited from getcontext() assuming that getcontext()
+ * is in the same shared object as _ctx_done().  For N32 and N64, GP
+ * is caller-save so will be preserved across the call to the callback
+ * function.  For O32, GP is callee-save, so save it in a different
+ * caller-save register (S1) while invoking the callback.  This is
+ * done instead of the usual SETUP_GP/SAVE_GP to avoid disturbing the
+ * stack frame setup by makecontext() for the callback function.
  */
 ENTRY(_ctx_start)
+#ifdef __mips_o32
+	move	s1, gp
+#endif
 	jalr	t9
 
+#ifdef __mips_o32
+	move	gp, s1
+#endif
 	move	a0, s0
 	PTR_LA	t9, _ctx_done
 	jalr	t9