On Thu, Sep 4, 2025 at 1:52 PM Peter Zijlstra <pet...@infradead.org> wrote: > > On Thu, Sep 04, 2025 at 01:49:49PM -0700, Andrii Nakryiko wrote: > > On Thu, Sep 4, 2025 at 1:35 PM Peter Zijlstra <pet...@infradead.org> wrote: > > > > > > On Thu, Sep 04, 2025 at 11:27:45AM -0700, Andrii Nakryiko wrote: > > > > > > > > > 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-jo...@kernel.org/
