MIPS: KGDB: Use kernel context for sleeping threads

Author: James Hogan <[email protected]> Thu Mar 30 16:06:02 2017 +0100
Comitter: Ralf Baechle <[email protected]> Wed Apr 12 22:23:03 2017 +0200
Commit: 4996462da4e166d61cdce7773c20e04faa33c0e8
Gitweb: https://git.linux-mips.org/g/ralf/linux/4996462da4e1
Branch: master

KGDB is a kernel debug stub and it can't be used to debug userland as it
can only safely access kernel memory.

On MIPS however KGDB has always got the register state of sleeping
processes from the userland register context at the beginning of the
kernel stack. This is meaningless for kernel threads (which never enter
userland), and for user threads it prevents the user seeing what it is
doing while in the kernel:

(gdb) info threads
  Id   Target Id         Frame
  ...
  3    Thread 2 (kthreadd) 0x0000000000000000 in ?? ()
  2    Thread 1 (init)   0x000000007705c4b4 in ?? ()
  1    Thread -2 (shadowCPU0) 0xffffffff8012524c in arch_kgdb_breakpoint () at arch/mips/kernel/kgdb.c:201

Get the register state instead from the (partial) kernel register
context stored in the task's thread_struct for resume() to restore. All
threads now correctly appear to be in context_switch():

(gdb) info threads
  Id   Target Id         Frame
  ...
  3    Thread 2 (kthreadd) context_switch (rq=<optimized out>, cookie=..., next=<optimized out>, prev=0x0) at kernel/sched/core.c:2903
  2    Thread 1 (init)   context_switch (rq=<optimized out>, cookie=..., next=<optimized out>, prev=0x0) at kernel/sched/core.c:2903
  1    Thread -2 (shadowCPU0) 0xffffffff8012524c in arch_kgdb_breakpoint () at arch/mips/kernel/kgdb.c:201

Call clobbered registers which aren't saved and exception registers
(BadVAddr & Cause) which can't be easily determined without stack
unwinding are reported as 0. The PC is taken from the return address,
such that the state presented matches that found immediately after
returning from resume().

Fixes: 8854700115ec ("[MIPS] kgdb: add arch support for the kernel's kgdb core")
Signed-off-by: James Hogan <[email protected]>
Cc: Jason Wessel <[email protected]>
Cc: [email protected]
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/15829/
Signed-off-by: Ralf Baechle <[email protected]>

---

 arch/mips/kernel/kgdb.c |   48 ++++++++++++++++++++++++++++++++---------------
 1 file changed, 33 insertions(+), 15 deletions(-)