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

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

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

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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.

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.