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    SubjectRe: [REGRESSION] 5.16rc1: SA_IMMUTABLE breaks debuggers
    On Wed, Nov 17, 2021 at 10:47:13AM -0800, Kyle Huey wrote:
    > rr, a userspace record and replay debugger[0], is completely broken on
    > 5.16rc1. I bisected this to 00b06da29cf9dc633cdba87acd3f57f4df3fd5c7.
    >
    > That patch makes two changes, it blocks sigaction from changing signal
    > handlers once the kernel has decided to force the program to take a
    > signal and it also stops notifying ptracers of the signal in the same
    > circumstances. The latter behavior is just wrong. There's no reason
    > that ptrace should not be able to observe and even change
    > (non-SIGKILL) forced signals. It should be reverted.
    >
    > This behavior change is also observable in gdb. If you take a program
    > that sets SIGSYS to SIG_IGN and then raises a SIGSYS via
    > SECCOMP_RET_TRAP and run it under gdb on a good kernel gdb will stop
    > when the SIGSYS is raised, let you inspect program state, etc. After
    > the SA_IMMUTABLE change gdb won't stop until the program has already
    > died of SIGSYS.

    Ah, hm, this was trying to fix the case where a program trips
    SECCOMP_RET_KILL (which is a "fatal SIGSYS"), and had been unobservable
    before. I guess the fix was too broad...

    --
    Kees Cook

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