gdb: set current thread in sparc_{fetch,collect}_inferior_registers (PR gdb/27147)

gdb: set current thread in sparc_{fetch,collect}_inferior_registers (PR gdb/27147)

PR 27147 shows that on sparc64, GDB is unable to properly unwind:

Expected result (from GDB 9.2):

    #0  0x0000000000108de4 in puts ()
    #1  0x0000000000100950 in hello () at gdb-test.c:4
    #2  0x0000000000100968 in main () at gdb-test.c:8

Actual result (from GDB latest git):

    #0  0x0000000000108de4 in puts ()
    #1  0x0000000000100950 in hello () at gdb-test.c:4
    Backtrace stopped: previous frame inner to this frame (corrupt stack?)

The first failing commit is 5b6d1e4fa4fc ("Multi-target support").  The cause
of the change in behavior is due to (thanks for Andrew Burgess for finding
this):

 - inferior_ptid is no longer set on entry of target_ops::wait, whereas
   it was set to something valid previously
 - deep down in linux_nat_target::wait (see stack trace below), we fetch
   the registers of the event thread
 - on sparc64, fetching registers involves reading memory (in
   sparc_supply_rwindow, see stack trace below)
 - reading memory (target_ops::xfer_partial) relies on inferior_ptid
   being set to the thread from which we want to read memory

This is where things go wrong:

    #0  linux_nat_target::xfer_partial (this=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, annex=0x0, readbuf=0x7feffe3b000 "", writebuf=0x0, offset=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:3697
    #1  0x00000100007f5b10 in raw_memory_xfer_partial (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, readbuf=0x7feffe3b000 "", writebuf=0x0, memaddr=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/target.c:912
    #2  0x00000100007f60e8 in memory_xfer_partial_1 (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, readbuf=0x7feffe3b000 "", writebuf=0x0, memaddr=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/target.c:1043
    #3  0x00000100007f61b4 in memory_xfer_partial (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, readbuf=0x7feffe3b000 "", writebuf=0x0, memaddr=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/target.c:1072
    #4  0x00000100007f6538 in target_xfer_partial (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, annex=0x0, readbuf=0x7feffe3b000 "", writebuf=0x0, offset=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/target.c:1129
    #5  0x00000100007f7094 in target_read_partial (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, annex=0x0, buf=0x7feffe3b000 "", offset=8791798050744, len=8, xfered_len=0x7feffe3ae88) at /home/simark/src/binutils-gdb/gdb/target.c:1375
    #6  0x00000100007f721c in target_read (ops=0x10000fa2c40 <the_sparc64_linux_nat_target>, object=TARGET_OBJECT_MEMORY, annex=0x0, buf=0x7feffe3b000 "", offset=8791798050744, len=8) at /home/simark/src/binutils-gdb/gdb/target.c:1415
    #7  0x00000100007f69d4 in target_read_memory (memaddr=8791798050744, myaddr=0x7feffe3b000 "", len=8) at /home/simark/src/binutils-gdb/gdb/target.c:1218
    #8  0x0000010000758520 in sparc_supply_rwindow (regcache=0x10000fea4f0, sp=8791798050736, regnum=-1) at /home/simark/src/binutils-gdb/gdb/sparc-tdep.c:1960
    #9  0x000001000076208c in sparc64_supply_gregset (gregmap=0x10000be3190 <sparc64_linux_ptrace_gregmap>, regcache=0x10000fea4f0, regnum=-1, gregs=0x7feffe3b230) at /home/simark/src/binutils-gdb/gdb/sparc64-tdep.c:1974
    #10 0x0000010000751b64 in sparc_fetch_inferior_registers (regcache=0x10000fea4f0, regnum=80) at /home/simark/src/binutils-gdb/gdb/sparc-nat.c:170
    #11 0x0000010000759d68 in sparc64_linux_nat_target::fetch_registers (this=0x10000fa2c40 <the_sparc64_linux_nat_target>, regcache=0x10000fea4f0, regnum=80) at /home/simark/src/binutils-gdb/gdb/sparc64-linux-nat.c:38
    #12 0x00000100008146ec in target_fetch_registers (regcache=0x10000fea4f0, regno=80) at /home/simark/src/binutils-gdb/gdb/target.c:3287
    #13 0x00000100006a8c5c in regcache::raw_update (this=0x10000fea4f0, regnum=80) at /home/simark/src/binutils-gdb/gdb/regcache.c:584
    #14 0x00000100006a8d94 in readable_regcache::raw_read (this=0x10000fea4f0, regnum=80, buf=0x7feffe3b7c0 "") at /home/simark/src/binutils-gdb/gdb/regcache.c:598
    #15 0x00000100006a93b8 in readable_regcache::cooked_read (this=0x10000fea4f0, regnum=80, buf=0x7feffe3b7c0 "") at /home/simark/src/binutils-gdb/gdb/regcache.c:690
    #16 0x00000100006b288c in readable_regcache::cooked_read<unsigned long, void> (this=0x10000fea4f0, regnum=80, val=0x7feffe3b948) at /home/simark/src/binutils-gdb/gdb/regcache.c:777
    #17 0x00000100006a9b44 in regcache_cooked_read_unsigned (regcache=0x10000fea4f0, regnum=80, val=0x7feffe3b948) at /home/simark/src/binutils-gdb/gdb/regcache.c:791
    #18 0x00000100006abf3c in regcache_read_pc (regcache=0x10000fea4f0) at /home/simark/src/binutils-gdb/gdb/regcache.c:1295
    #19 0x0000010000507920 in save_stop_reason (lp=0x10000fc5b10) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:2612
    #20 0x00000100005095a4 in linux_nat_filter_event (lwpid=520983, status=1407) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:3050
    #21 0x0000010000509f9c in linux_nat_wait_1 (ptid=..., ourstatus=0x7feffe3c8f0, target_options=...) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:3194
    #22 0x000001000050b1d0 in linux_nat_target::wait (this=0x10000fa2c40 <the_sparc64_linux_nat_target>, ptid=..., ourstatus=0x7feffe3c8f0, target_options=...) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:3432
    #23 0x00000100007f8ac0 in target_wait (ptid=..., status=0x7feffe3c8f0, options=...) at /home/simark/src/binutils-gdb/gdb/target.c:2000
    #24 0x00000100004ac17c in do_target_wait_1 (inf=0x1000116d280, ptid=..., status=0x7feffe3c8f0, options=...) at /home/simark/src/binutils-gdb/gdb/infrun.c:3464
    #25 0x00000100004ac3b8 in operator() (__closure=0x7feffe3c678, inf=0x1000116d280) at /home/simark/src/binutils-gdb/gdb/infrun.c:3527
    #26 0x00000100004ac7cc in do_target_wait (wait_ptid=..., ecs=0x7feffe3c8c8, options=...) at /home/simark/src/binutils-gdb/gdb/infrun.c:3540
    #27 0x00000100004ad8c4 in fetch_inferior_event () at /home/simark/src/binutils-gdb/gdb/infrun.c:3880
    #28 0x0000010000485568 in inferior_event_handler (event_type=INF_REG_EVENT) at /home/simark/src/binutils-gdb/gdb/inf-loop.c:42
    #29 0x000001000050d394 in handle_target_event (error=0, client_data=0x0) at /home/simark/src/binutils-gdb/gdb/linux-nat.c:4060
    #30 0x0000010000ab5c8c in handle_file_event (file_ptr=0x10001207270, ready_mask=1) at /home/simark/src/binutils-gdb/gdbsupport/event-loop.cc:575
    #31 0x0000010000ab6334 in gdb_wait_for_event (block=0) at /home/simark/src/binutils-gdb/gdbsupport/event-loop.cc:701
    #32 0x0000010000ab487c in gdb_do_one_event () at /home/simark/src/binutils-gdb/gdbsupport/event-loop.cc:212
    #33 0x0000010000542668 in start_event_loop () at /home/simark/src/binutils-gdb/gdb/main.c:348
    #34 0x000001000054287c in captured_command_loop () at /home/simark/src/binutils-gdb/gdb/main.c:408
    #35 0x0000010000544e84 in captured_main (data=0x7feffe3d188) at /home/simark/src/binutils-gdb/gdb/main.c:1242
    #36 0x0000010000544f2c in gdb_main (args=0x7feffe3d188) at /home/simark/src/binutils-gdb/gdb/main.c:1257
    #37 0x00000100000c1f14 in main (argc=4, argv=0x7feffe3d548) at /home/simark/src/binutils-gdb/gdb/gdb.c:32

There is a target_read_memory call in sparc_supply_rwindow, whose return
value is not checked.  That call fails, because inferior_ptid does not
contain a valid ptid, and uninitialized buffer contents is used.
Ultimately it results in a corrupt stop_pc.

target_ops::fetch_registers can be (and should remain, in my opinion)
independent of inferior_ptid, because the ptid of the thread from which
to fetch registers can be obtained from the regcache.  In other words,
implementations of target_ops::fetch_registers should not rely on
inferior_ptid having a sensible value on entry.

The sparc64_linux_nat_target::fetch_registers case is special, because it calls
a target method that is dependent on the inferior_ptid value
(target_read_inferior, and ultimately target_ops::xfer_partial).  So I would
say it's the responsibility of sparc64_linux_nat_target::fetch_registers to set
up inferior_ptid correctly prior to calling target_read_inferior.

This patch makes sparc64_linux_nat_target::fetch_registers (and
store_registers, since it works the same) temporarily set inferior_ptid.  If we
ever make target_ops::xfer_partial independent of inferior_ptid, setting
inferior_ptid won't be necessary, we'll simply pass down the ptid as a
parameter in some way.

I chose to set/restore inferior_ptid in sparc_fetch_inferior_registers, because
I am not convinced that doing so in an inner location (in sparc_supply_rwindow
for instance) would always be correct.  We have access to the ptid in
sparc_supply_rwindow (from the regcache), so we _could_ set inferior_ptid
there.  However, I don't want to just set inferior_ptid, as that would make it
not desync'ed with `current_thread ()` and `current_inferior ()`.  It's
preferable to use switch_to_thread instead, as that switches all the global
"current" stuff in a coherent way.  But doing so requires a `thread_info *`,
and getting a `thread_info *` from a ptid requires a `process_stratum_target
*`.  We could use `current_inferior()->process_target()` in
sparc_supply_rwindow for this (using target_read_memory uses the current
inferior's target stack anyway).  However, sparc_supply_rwindow is also used in
the context of BSD uthreads, where a thread stratum target defines threads.  I
presume the ptid in the regcache would be the ptid of the uthread, defined by
the thread stratum target (bsd_uthread_target).  Using
`current_inferior()->process_target()` would look up a ptid defined by the
thread stratum target using the process stratum target.  I don't think it would
give good results.  So I prefer playing it safe and looking up the thread
earlier, in sparc_fetch_inferior_registers.

I added some assertions (in sparc_supply_rwindow and others) to verify
that the regcache's ptid matches inferior_ptid.  That verifies that the
caller has properly set the correct global context.  This would have
caught (though a failed assertion) the current problem.

gdb/ChangeLog:

	PR gdb/27147
	* sparc-nat.h (sparc_fetch_inferior_registers): Add
	process_stratum_target parameter,
	sparc_store_inferior_registers): update callers.
	* sparc-nat.c (sparc_fetch_inferior_registers,
	sparc_store_inferior_registers): Add process_stratum_target
	parameter.  Switch current thread before calling
	sparc_supply_gregset / sparc_collect_rwindow.
	(sparc_store_inferior_registers): Likewise.
	* sparc-obsd-tdep.c (sparc32obsd_supply_uthread): Add assertion.
	(sparc32obsd_collect_uthread): Likewise.
	* sparc-tdep.c (sparc_supply_rwindow, sparc_collect_rwindow):
	Add assertion.
	* sparc64-obsd-tdep.c (sparc64obsd_collect_uthread,
	sparc64obsd_supply_uthread): Add assertion.

Change-Id: I16c658cd70896cea604516714f7e2428fbaf4301