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List:       fedora-devel-list
Subject:    Re: F37 proposal: Add -fno-omit-frame-pointer to default compilation flags (System-Wide Change propo
From:       Demi Marie Obenour <demiobenour () gmail ! com>
Date:       2022-11-07 21:41:49
Message-ID: 6a7b5746-11ba-b532-9fa0-959b065b5631 () gmail ! com
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On 11/7/22 09:59, Daniel Alley wrote:
> The references of the proposal has some discussion on this subject.
> Basically, this was already considered and rejected by the kernel
> developers including Torvalds directly on the basis that DWARF
> unwinding support is simply too complex and nonperformant to be
> allowed in the kernel.

Three other options I can think of:

1. Use an out-of-tree kernel module, as systemtap does.
2. Find a better unwind information format for userspace.
3. Do the actual unwinding in userspace in the context of the process
   being profiled.

The last option can be implemented by having the unwind code be in
the vDSO.  This requires that the kernel inject an asynchronous
interrupt to userspace, but that is no different than calling
a signal handler.  This also solves other problems: bugs in the
unwinder do not compromise or crash the kernel, and the unwinding
code can take page faults if necessary.

Even with -fno-omit-frame-pointer, frame pointer unwinding will
still be unreliable.  There is a significant amount of assembler
code in e.g. glibc that does not maintain a frame pointer, and I
suspect the same is true for OpenSSL and other libraries.  I am
also not sure if OCaml supports generating frame pointers, and I
know that GHC does not.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
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