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    x86/unwind: Use MSB for frame pointer encoding on 32-bit · 5c99b692
    Josh Poimboeuf authored
    
    
    On x86-32, Tetsuo Handa and Fengguang Wu reported unwinder warnings
    like:
    
      WARNING: kernel stack regs at f60bb9c8 in swapper:1 has bad 'bp' value 0ba00000
    
    And also there were some stack dumps with a bunch of unreliable '?'
    symbols after an apic_timer_interrupt symbol, meaning the unwinder got
    confused when it tried to read the regs.
    
    The cause of those issues is that, with GCC 4.8 (and possibly older),
    there are cases where GCC misaligns the stack pointer in a leaf function
    for no apparent reason:
    
      c124a388 <acpi_rs_move_data>:
      c124a388:       55                      push   %ebp
      c124a389:       89 e5                   mov    %esp,%ebp
      c124a38b:       57                      push   %edi
      c124a38c:       56                      push   %esi
      c124a38d:       89 d6                   mov    %edx,%esi
      c124a38f:       53                      push   %ebx
      c124a390:       31 db                   xor    %ebx,%ebx
      c124a392:       83 ec 03                sub    $0x3,%esp
      ...
      c124a3e3:       83 c4 03                add    $0x3,%esp
      c124a3e6:       5b                      pop    %ebx
      c124a3e7:       5e                      pop    %esi
      c124a3e8:       5f                      pop    %edi
      c124a3e9:       5d                      pop    %ebp
      c124a3ea:       c3                      ret
    
    If an interrupt occurs in such a function, the regs on the stack will be
    unaligned, which breaks the frame pointer encoding assumption.  So on
    32-bit, use the MSB instead of the LSB to encode the regs.
    
    This isn't an issue on 64-bit, because interrupts align the stack before
    writing to it.
    
    Reported-and-tested-by: default avatarTetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Reported-and-tested-by: default avatarFengguang Wu <fengguang.wu@intel.com>
    Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    Cc: Byungchul Park <byungchul.park@lge.com>
    Cc: LKP <lkp@01.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: http://lkml.kernel.org/r/279a26996a482ca716605c7dbc7f2db9d8d91e81.1507597785.git.jpoimboe@redhat.com
    
    
    Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
    5c99b692