• Linus Torvalds's avatar
    vm: add VM_FAULT_SIGSEGV handling support · 33692f27
    Linus Torvalds authored
    The core VM already knows about VM_FAULT_SIGBUS, but cannot return a
    "you should SIGSEGV" error, because the SIGSEGV case was generally
    handled by the caller - usually the architecture fault handler.
    
    That results in lots of duplication - all the architecture fault
    handlers end up doing very similar "look up vma, check permissions, do
    retries etc" - but it generally works.  However, there are cases where
    the VM actually wants to SIGSEGV, and applications _expect_ SIGSEGV.
    
    In particular, when accessing the stack guard page, libsigsegv expects a
    SIGSEGV.  And it usually got one, because the stack growth is handled by
    that duplicated architecture fault handler.
    
    However, when the generic VM layer started propagating the error return
    from the stack expansion in commit fee7e49d ("mm: propagate error
    from stack expansion even for guard page"), that now exposed the
    existing VM_FAULT_SIGBUS result to user space.  And user space really
    expected SIGSEGV, not SIGBUS.
    
    To fix that case, we need to add a VM_FAULT_SIGSEGV, and teach all those
    duplicate architecture fault handlers about it.  They all already have
    the code to handle SIGSEGV, so it's about just tying that new return
    value to the existing code, but it's all a bit annoying.
    
    This is the mindless minimal patch to do this.  A more extensive patch
    would be to try to gather up the mostly shared fault handling logic into
    one generic helper routine, and long-term we really should do that
    cleanup.
    
    Just from this patch, you can generally see that most architectures just
    copied (directly or indirectly) the old x86 way of doing things, but in
    the meantime that original x86 model has been improved to hold the VM
    semaphore for shorter times etc and to handle VM_FAULT_RETRY and other
    "newer" things, so it would be a good idea to bring all those
    improvements to the generic case and teach other architectures about
    them too.
    Reported-and-tested-by: default avatarTakashi Iwai <tiwai@suse.de>
    Tested-by: default avatarJan Engelhardt <jengelh@inai.de>
    Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com> # "s390 still compiles and boots"
    Cc: linux-arch@vger.kernel.org
    Cc: stable@vger.kernel.org
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    33692f27
fault.c 6.34 KB