[PATCH v3 16/17] S/390: displaced stepping and PC-relative RIL-b/RIL-c instructions

Pedro Alves
2015-04-17T11:06:01Z

This adds displaced stepping support for the General-Instruction
Extension Facility instructions, which have a PC-relative displacement
(RIL-b/RIL-c). We already handle RIL branches, but not others.

Currently, displaced stepping a breakpoint put on any of these
instructions results in the inferior crashing when or after the
instruction is executed out-of-line in the scratch pad.

This patch takes the easy route of patching the displacement in the
copy of the instruction in the scratch pad. As the displacement is a
signed 32-bit field, it's possible that the stratch pad ends too far
that the needed displacement doesn't fit in the adjusted instruction,
as e.g., if stepping over a breakpoint in a shared library (the
scratch pad is around the main program's entry point). That case is
detected and GDB falls back to stepping over the breakpoint in-line
(which involves pausing all threads momentarily).

(We could probably do something smarter, but I don't plan on doing it
myself. This was already sufficient to get "maint set target-non-stop
on" working regression free on S/390.)

Tested on S/390 RHEL 7.1, where it fixes a few hundred FAILs when
testing with displaced stepping force-enabled, with the end result
being no regressions compared to a test run that doesn't force
displaced stepping. Fixes the non-stop tests compared to mainline
too; most are crashing due to this on the machine I run tests on.

gdb/ChangeLog:
2015-04-17 Pedro Alves <palves@redhat.com>

* s390-linux-tdep.c (is_non_branch_ril)
(s390_displaced_step_copy_insn): New functions.
(s390_displaced_step_fixup): Update comment.
(s390_gdbarch_init): Install s390_displaced_step_copy_insn as
gdbarch_displaced_step_copy_insn hook.

v3:

- No changes.
---
gdb/s390-linux-tdep.c | 115 ++++++++++++++++++++++++++++++++++++++++++++++++--
1 file changed, 112 insertions(+), 3 deletions(-)

diff --git a/gdb/s390-linux-tdep.c b/gdb/s390-linux-tdep.c
index b83a264..41790a4 100644
--- a/gdb/s390-linux-tdep.c
+++ b/gdb/s390-linux-tdep.c
@@ -1529,6 +1529,116 @@ s390_in_function_epilogue_p (struct gdbarch *gdbarch, CORE_ADDR pc)

/* Displaced stepping. */

+/* Return true if INSN is a non-branch RIL-b or RIL-c format
+ instruction. */
+
+static int
+is_non_branch_ril (gdb_byte *insn)
+{
+ gdb_byte op1 = insn[0];
+
+ if (op1 == 0xc4)
+ {
+ gdb_byte op2 = insn[1] & 0x0f;
+
+ switch (op2)
+ {
+ case 0x02: /* llhrl */
+ case 0x04: /* lghrl */
+ case 0x05: /* lhrl */
+ case 0x06: /* llghrl */
+ case 0x07: /* sthrl */
+ case 0x08: /* lgrl */
+ case 0x0b: /* stgrl */
+ case 0x0c: /* lgfrl */
+ case 0x0d: /* lrl */
+ case 0x0e: /* llgfrl */
+ case 0x0f: /* strl */
+ return 1;
+ }
+ }
+ else if (op1 == 0xc6)
+ {
+ gdb_byte op2 = insn[1] & 0x0f;
+
+ switch (op2)
+ {
+ case 0x00: /* exrl */
+ case 0x02: /* pfdrl */
+ case 0x04: /* cghrl */
+ case 0x05: /* chrl */
+ case 0x06: /* clghrl */
+ case 0x07: /* clhrl */
+ case 0x08: /* cgrl */
+ case 0x0a: /* clgrl */
+ case 0x0c: /* cgfrl */
+ case 0x0d: /* crl */
+ case 0x0e: /* clgfrl */
+ case 0x0f: /* clrl */
+ return 1;
+ }
+ }
+
+ return 0;
+}
+
+/* Implementation of gdbarch_displaced_step_copy_insn. */
+
+static struct displaced_step_closure *
+s390_displaced_step_copy_insn (struct gdbarch *gdbarch,
+ CORE_ADDR from, CORE_ADDR to,
+ struct regcache *regs)
+{
+ size_t len = gdbarch_max_insn_length (gdbarch);
+ gdb_byte *buf = xmalloc (len);
+ struct cleanup *old_chain = make_cleanup (xfree, buf);
+
+ read_memory (from, buf, len);
+
+ /* Adjust the displacement field of PC-relative RIL instructions,
+ except branches. The latter are handled in the fixup hook. */
+ if (is_non_branch_ril (buf))
+ {
+ LONGEST offset;
+
+ offset = extract_signed_integer (buf + 2, 4, BFD_ENDIAN_BIG);
+ offset = (from - to + offset * 2) / 2;
+
+ /* If the instruction is too far from the jump pad, punt. This
+ will usually happen with instructions in shared libraries.
+ We could probably support these by rewriting them to be
+ absolute or fully emulating them. */
+ if (offset < INT32_MIN || offset > INT32_MAX)
+ {
+ /* Let the core fall back to stepping over the breakpoint
+ in-line. */
+ if (debug_displaced)
+ {
+ fprintf_unfiltered (gdb_stdlog,
+ "displaced: can't displaced step "
+ "RIL instruction: offset %s out of range\n",
+ plongest (offset));
+ }
+ do_cleanups (old_chain);
+ return NULL;
+ }
+
+ store_signed_integer (buf + 2, 4, BFD_ENDIAN_BIG, offset);
+ }
+
+ write_memory (to, buf, len);
+
+ if (debug_displaced)
+ {
+ fprintf_unfiltered (gdb_stdlog, "displaced: copy %s->%s: ",
+ paddress (gdbarch, from), paddress (gdbarch, to));
+ displaced_step_dump_bytes (gdb_stdlog, buf, len);
+ }
+
+ discard_cleanups (old_chain);
+ return (struct displaced_step_closure *) buf;
+}
+
/* Fix up the state of registers and memory after having single-stepped
a displaced instruction. */
static void
@@ -1537,8 +1647,7 @@ s390_displaced_step_fixup (struct gdbarch *gdbarch,
CORE_ADDR from, CORE_ADDR to,
struct regcache *regs)
{
- /* Since we use simple_displaced_step_copy_insn, our closure is a
- copy of the instruction. */
+ /* Our closure is a copy of the instruction. */
gdb_byte *insn = (gdb_byte *) closure;
static int s390_instrlen[] = { 2, 4, 4, 6 };
int insnlen = s390_instrlen[insn[0] >> 6];
@@ -3291,7 +3400,7 @@ s390_gdbarch_init (struct gdbarch_info info, struct gdbarch_list *arches)

/* Displaced stepping. */
set_gdbarch_displaced_step_copy_insn (gdbarch,
- simple_displaced_step_copy_insn);
+ s390_displaced_step_copy_insn);
set_gdbarch_displaced_step_fixup (gdbarch, s390_displaced_step_fixup);
set_gdbarch_displaced_step_free_closure (gdbarch,
simple_displaced_step_free_closure);
--
1.9.3