Re: [REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Linus Torvalds]

On Thu, 23 Apr 2026 at 10:19, Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
>   C) Declare that this is not a regression because the ABI guarantee is
>      not violated and the RO property has been enforcable by RSEQ
>      debugging since day one.

No, if this actually hits real users, that is not an option. If real
users never used RSEQ debugging options, those options are simply
irrelevant.

Regression rules have never been about "it wouldn't have worked in
some other configuration". That's like saying "that code would never
have worked on another architecture". It may be true, but it's
irrelevant for the people whose binaries no longer work.

We will have to fix this.

This is not some kind of gray area. It clearly violates our regression rules.

The only "ABI guarantee" is what people actually see and use, not some
debug option that wasn't enabled.

And I just checked - it's not enabled in at least the Fedora distro
kernels. Presumably other distros don't enable it either. So no actual
non-kernel developer would *ever* have hit it, and claiming it is
relevant is just garbage.

IOW, that debug option was always a complete no-op except for kernel developers.

In fact, that debug option is actively *hidden* - you have to enable
EXPERT to even see it. Soi it really is not a real option for normal
people AT ALL.

Christ, even *I* don't enable EXPERT except for build testing. It's
literally something that only embedded people doing odd things should
do.

If that rule was actually an important part of the ABI, it shouldn't
have been a debug thing.

So:

 (a) the debug code in question needs to just be removed, since it's
now actively detrimental, and means that any kernel developer who
*does* enable it can't actually test this case any more. It's checking
for something that has been shown to not be true.

 (b) we need to fix this (revert if it can't be fixed otherwise)

I see some patches flying around, but am not clear on whether there
was an actual patch that make this work again?

             Linus