• James Hogan's avatar
    MIPS: KGDB: Use kernel context for sleeping threads · c38cb25b
    James Hogan authored
    commit 162b270c upstream.
    
    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: 88547001 ("[MIPS] kgdb: add arch support for the kernel's kgdb core")
    Signed-off-by: default avatarJames Hogan <james.hogan@imgtec.com>
    Cc: Jason Wessel <jason.wessel@windriver.com>
    Cc: linux-mips@linux-mips.org
    Patchwork: https://patchwork.linux-mips.org/patch/15829/Signed-off-by: default avatarRalf Baechle <ralf@linux-mips.org>
    Signed-off-by: default avatarBen Hutchings <ben@decadent.org.uk>
    c38cb25b
kgdb.c 11.4 KB