• Russ Cox's avatar
    runtime: test malformed address fault and fix on OS X · 17f9423e
    Russ Cox authored
    The garbage collector poison pointers
    (0x6969696969696969 and 0x6868686868686868)
    are malformed addresses on amd64.
    That is, they are not 48-bit addresses sign extended
    to 64 bits. This causes a different kind of hardware fault
    than the usual 'unmapped page' when accessing such
    an address, and OS X 10.9.2 sends the resulting SIGSEGV
    incorrectly, making it look like it was user-generated
    rather than kernel-generated and does not include the
    faulting address. This means that in GODEBUG=gcdead=1
    mode, if there is a bug and something tries to dereference
    a poisoned pointer, the runtime delivers the SIGSEGV to
    os/signal and returns to the faulting code, which faults
    again, causing the process to hang instead of crashing.
    
    Fix by rewriting "user-generated" SIGSEGV on OS X to
    look like a kernel-generated SIGSEGV with fault address
    0xb01dfacedebac1e.
    
    I chose that address because (1) when printed in hex
    during a crash, it is obviously spelling out English text,
    (2) there are no current Google hits for that pointer,
    which will make its origin easy to find once this CL
    is indexed, and (3) it is not an altogether inaccurate
    description of the situation.
    
    Add a test. Maybe other systems will break too.
    
    LGTM=khr
    R=golang-codereviews, khr
    CC=golang-codereviews, iant, ken
    https://golang.org/cl/83270049
    17f9423e
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