Hi Andrii, Sorry I excavated this from patchwork. On Mon, 29 Apr 2024 15:38:08 -0700 Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > On Wed, Apr 24, 2024 at 5:02 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote: > > > > At the lowest level, rethook-based kretprobes on x86-64 architecture go > > through arch_rethoook_trampoline() function, manually written in > > assembly, which calls into a simple arch_rethook_trampoline_callback() > > function, written in C, and only doing a few straightforward field > > assignments, before calling further into rethook_trampoline_handler(), > > which handles kretprobe callbacks generically. > > > > Looking at simplicity of arch_rethook_trampoline_callback(), it seems > > not really worthwhile to spend an extra function call just to do 4 or > > 5 assignments. As such, this patch proposes to "inline" > > arch_rethook_trampoline_callback() into arch_rethook_trampoline() by > > manually implementing it in an assembly code. Yeah, I think it is possible, but this makes code ugly, that is trade-off. As you say, we should move this with other ugly inline assembly code into kprobe.S or something like it. With my current fprobe-on-fgraph, rethook is only required for kretprobes, so it is natual and simple to have kprobe.S. Thank you, > > > > This has two motivations. First, we do get a bit of runtime speed up by > > avoiding function calls. Using BPF selftests's bench tool, we see > > 0.6%-0.8% throughput improvement for kretprobe/multi-kretprobe > > triggering code path: > > > > BEFORE (latest probes/for-next) > > =============================== > > kretprobe : 10.455 ± 0.024M/s > > kretprobe-multi: 11.150 ± 0.012M/s > > > > AFTER (probes/for-next + this patch) > > ==================================== > > kretprobe : 10.540 ± 0.009M/s (+0.8%) > > kretprobe-multi: 11.219 ± 0.042M/s (+0.6%) > > > > Second, and no less importantly for some specialized use cases, this > > avoids unnecessarily "polluting" LBR records with an extra function call > > (recorded as a jump by CPU). This is the case for the retsnoop ([0]) > > tool, which relies havily on capturing LBR records to provide users with > > lots of insight into kernel internals. > > > > This RFC patch is only inlining this function for x86-64, but it's > > possible to do that for 32-bit x86 arch as well and then remove > > arch_rethook_trampoline_callback() implementation altogether. Please let > > me know if this change is acceptable and whether I should complete it > > with 32-bit "inlining" as well. Thanks! > > > > [0] https://nakryiko.com/posts/retsnoop-intro/#peering-deep-into-functions-with-lbr > > > > Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > --- > > arch/x86/kernel/asm-offsets_64.c | 4 ++++ > > arch/x86/kernel/rethook.c | 37 +++++++++++++++++++++++++++----- > > 2 files changed, 36 insertions(+), 5 deletions(-) > > > > diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c > > index bb65371ea9df..5c444abc540c 100644 > > --- a/arch/x86/kernel/asm-offsets_64.c > > +++ b/arch/x86/kernel/asm-offsets_64.c > > @@ -42,6 +42,10 @@ int main(void) > > ENTRY(r14); > > ENTRY(r15); > > ENTRY(flags); > > + ENTRY(ip); > > + ENTRY(cs); > > + ENTRY(ss); > > + ENTRY(orig_ax); > > BLANK(); > > #undef ENTRY > > > > diff --git a/arch/x86/kernel/rethook.c b/arch/x86/kernel/rethook.c > > index 8a1c0111ae79..3e1c01beebd1 100644 > > --- a/arch/x86/kernel/rethook.c > > +++ b/arch/x86/kernel/rethook.c > > @@ -6,6 +6,7 @@ > > #include <linux/rethook.h> > > #include <linux/kprobes.h> > > #include <linux/objtool.h> > > +#include <asm/asm-offsets.h> > > > > #include "kprobes/common.h" > > > > @@ -34,10 +35,36 @@ asm( > > " pushq %rsp\n" > > " pushfq\n" > > SAVE_REGS_STRING > > - " movq %rsp, %rdi\n" > > - " call arch_rethook_trampoline_callback\n" > > + " movq %rsp, %rdi\n" /* $rdi points to regs */ > > + /* fixup registers */ > > + /* regs->cs = __KERNEL_CS; */ > > + " movq $" __stringify(__KERNEL_CS) ", " __stringify(pt_regs_cs) "(%rdi)\n" > > + /* regs->ip = (unsigned long)&arch_rethook_trampoline; */ > > + " movq $arch_rethook_trampoline, " __stringify(pt_regs_ip) "(%rdi)\n" > > + /* regs->orig_ax = ~0UL; */ > > + " movq $0xffffffffffffffff, " __stringify(pt_regs_orig_ax) "(%rdi)\n" > > + /* regs->sp += 2*sizeof(long); */ > > + " addq $16, " __stringify(pt_regs_sp) "(%rdi)\n" > > + /* 2nd arg is frame_pointer = (long *)(regs + 1); */ > > + " lea " __stringify(PTREGS_SIZE) "(%rdi), %rsi\n" > > BTW, all this __stringify() ugliness can be avoided if we move this > assembly into its own .S file, like lots of other assembly functions > in arch/x86/kernel subdir. That has another benefit of generating > better line information in DWARF for those assembly instructions. It's > lots more work, so before I do this, I'd like to get confirmation that > this change is acceptable in principle. > > > + /* > > + * The return address at 'frame_pointer' is recovered by the > > + * arch_rethook_fixup_return() which called from this > > + * rethook_trampoline_handler(). > > + */ > > + " call rethook_trampoline_handler\n" > > + /* > > + * Copy FLAGS to 'pt_regs::ss' so we can do RET right after POPF. > > + * > > + * We don't save/restore %rax below, because we ignore > > + * rethook_trampoline_handler result. > > + * > > + * *(unsigned long *)®s->ss = regs->flags; > > + */ > > + " mov " __stringify(pt_regs_flags) "(%rsp), %rax\n" > > + " mov %rax, " __stringify(pt_regs_ss) "(%rsp)\n" > > RESTORE_REGS_STRING > > - /* In the callback function, 'regs->flags' is copied to 'regs->ss'. */ > > + /* We just copied 'regs->flags' into 'regs->ss'. */ > > " addq $16, %rsp\n" > > " popfq\n" > > #else > > @@ -61,6 +88,7 @@ asm( > > ); > > NOKPROBE_SYMBOL(arch_rethook_trampoline); > > > > +#ifdef CONFIG_X86_32 > > /* > > * Called from arch_rethook_trampoline > > */ > > @@ -70,9 +98,7 @@ __used __visible void arch_rethook_trampoline_callback(struct pt_regs *regs) > > > > /* fixup registers */ > > regs->cs = __KERNEL_CS; > > -#ifdef CONFIG_X86_32 > > regs->gs = 0; > > -#endif > > regs->ip = (unsigned long)&arch_rethook_trampoline; > > regs->orig_ax = ~0UL; > > regs->sp += 2*sizeof(long); > > @@ -92,6 +118,7 @@ __used __visible void arch_rethook_trampoline_callback(struct pt_regs *regs) > > *(unsigned long *)®s->ss = regs->flags; > > } > > NOKPROBE_SYMBOL(arch_rethook_trampoline_callback); > > +#endif > > > > /* > > * arch_rethook_trampoline() skips updating frame pointer. The frame pointer > > -- > > 2.43.0 > > -- Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
