[Lldb-commits] [PATCH] D98529: [lldb] Strip pointer authentication codes from aarch64 pc.

[Justin Cohen via Phabricator via lldb-commits]

justincohen added a comment.



> Clearing PAC bits is a little more complicated than just clearing the bits, though.  Bit 55 tells us whether the high bits are all 0's or all 1's (on Darwin, in EL0 processes they're all 0's, in EL1, all 1's).  If we had a setting to provide a mask instead of the number of bits that are valid in addressing, that might lead someone to try to use it for a different purpose.  Trying to imagine a scenario like this, maybe someone could know that a certain range of the address space isn't used for a certain type of pointer, and that they could reuse those bits as a Top Byte Ignore kind of thing, but the generated code would need to clear/set those bits before dereferencing, or we'd need a CPU with that kind of capability.  Maybe there could be examples of this today like the thumb bit on armv7, where the 0th bit on something with alignment restrictions can be used to carry metadata, although I can't think of anything like that on AArch/x86_64 (the only two targets I can really remember well these days).

Copying over a comment from pcc@ on the minidump change here: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2773358/5//COMMIT_MSG#15 :
 > On Linux, the mask that you get from NT_ARM_PAC_MASK specifies which bits need to be cleared from the pointer. So to use the mask, you AND with the inverse.
 > This also has the advantage that 0 is not a special case, since you can AND with its inverse and get the same pointer back.

What if we invert the mask, and do something like instead?
`(ptr & (1ULL << 55)) ? (ptr | mask) : (ptr & ~mask);`

This should be very similar to the pseudocode here: https://webcache.googleusercontent.com/search?q=cache:3fCUm601caMJ:https://developer.arm.com/ja/docs/ddi0596/latest/shared-pseudocode-functions/aarch64-functionspac-pseudocode+&cd=2&hl=en&ct=clnk&gl=us


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https://reviews.llvm.org/D98529