On Thu, Sep 4, 2025 at 1:52 PM Peter Zijlstra [off-list ref] wrote:

On Thu, Sep 04, 2025 at 01:49:49PM -0700, Andrii Nakryiko wrote:

quoted

On Thu, Sep 4, 2025 at 1:35 PM Peter Zijlstra [off-list ref] wrote:

quoted

On Thu, Sep 04, 2025 at 11:27:45AM -0700, Andrii Nakryiko wrote:

quoted

quoted

quoted

So I've been thinking what's the simplest and most reliable way to
feature-detect support for this sys_uprobe (e.g., for libbpf to know
whether we should attach at nop5 vs nop1), and clearly that would be

wrt nop5/nop1.. so the idea is to have USDT macro emit both nop1,nop5
and store some info about that in the usdt's elf note, right?

Wait, what? You're doing to emit 6 bytes and two nops? Why? Surely the
old kernel can INT3 on top of a NOP5?

Yes it can, but it's 2x slower in terms of uprobe triggering compared
to nop1.

Why? That doesn't really make sense.

Of course it's silly... It's because nop5 wasn't recognized as one of
the emulated instructions, so was handled through single-stepping.

I realize its probably to late to fix the old kernel not to be stupid --
this must be something stupid, right? But now I need to know.

Jiri fixed this, but as you said, too late for old kernels. See [0]
for the patch that landed not so long ago.

  [0] https://lore.kernel.org/all/20250414083647.1234007-1-jolsa@kernel.org/ (local)