Re: [PATCH v4 5/9] ftrace: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 25 Aug 2023 14:49:48 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Wed, Aug 23, 2023 at 8:16 AM Masami Hiramatsu (Google)
> <mhiramat@kernel.org> wrote:
> >
> > From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >
> > Add ftrace_partial_regs() which converts the ftrace_regs to pt_regs.
> > If the architecture defines its own ftrace_regs, this copies partial
> > registers to pt_regs and returns it. If not, ftrace_regs is the same as
> > pt_regs and ftrace_partial_regs() will return ftrace_regs::regs.
> >
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> > Acked-by: Florent Revest <revest@chromium.org>
> > ---
> >  Changes in v3:
> >   - Fix to use pt_regs::regs instead of x.
> >   - Return ftrace_regs::regs forcibly if HAVE_PT_REGS_COMPAT_FTRACE_REGS=y.
> >   - Fix typo.
> >   - Fix to copy correct registers to the pt_regs on arm64.
> >  Changes in v4:
> >   - Change the patch order in the series so that fprobe event can use this.
> > ---
> >  arch/arm64/include/asm/ftrace.h |   11 +++++++++++
> >  include/linux/ftrace.h          |   17 +++++++++++++++++
> >  2 files changed, 28 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> > index ab158196480c..5ad24f315d52 100644
> > --- a/arch/arm64/include/asm/ftrace.h
> > +++ b/arch/arm64/include/asm/ftrace.h
> > @@ -137,6 +137,17 @@ ftrace_override_function_with_return(struct ftrace_regs *fregs)
> >         fregs->pc = fregs->lr;
> >  }
> >
> > +static __always_inline struct pt_regs *
> > +ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
> > +{
> > +       memcpy(regs->regs, fregs->regs, sizeof(u64) * 9);
> > +       regs->sp = fregs->sp;
> > +       regs->pc = fregs->pc;
> > +       regs->regs[29] = fregs->fp;
> > +       regs->regs[30] = fregs->lr;
> 
> I see that orig_x0 from pt_regs is used on arm64 to get syscall's
> first parameter. And it seems like this ftrace_regs to pt_regs
> conversion doesn't touch orig_x0 at all. Is it maintained/preserved
> somewhere else, or will we lose the ability to get the first syscall's
> argument?

Thanks for checking it!

Does BPF uses kprobe probe to trace syscalls? Since we have raw_syscall
trace events, no need to use kprobe to do that. (and I don't recommend to
use kprobe to do such fixed event)

> 
> Looking at libbpf's bpf_tracing.h, other than orig_x0, I think all the
> other registers are still preserved, so this seems to be the only
> potential problem.

Great!

Thank you,

> 
> 
> > +       return regs;
> > +}
> > +
> >  int ftrace_regs_query_register_offset(const char *name);
> >
> >  int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
> > diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> > index c0a42d0860b8..a6ed2aa71efc 100644
> > --- a/include/linux/ftrace.h
> > +++ b/include/linux/ftrace.h
> > @@ -165,6 +165,23 @@ static __always_inline struct pt_regs *ftrace_get_regs(struct ftrace_regs *fregs
> >         return arch_ftrace_get_regs(fregs);
> >  }
> >
> > +#if !defined(CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS) || \
> > +       defined(CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST)
> > +
> > +static __always_inline struct pt_regs *
> > +ftrace_partial_regs(struct ftrace_regs *fregs, struct pt_regs *regs)
> > +{
> > +       /*
> > +        * If CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST=y, ftrace_regs memory
> > +        * layout is the same as pt_regs. So always returns that address.
> > +        * Since arch_ftrace_get_regs() will check some members and may return
> > +        * NULL, we can not use it.
> > +        */
> > +       return &fregs->regs;
> > +}
> > +
> > +#endif /* !CONFIG_HAVE_DYNAMIC_FTRACE_WITH_ARGS || CONFIG_HAVE_PT_REGS_TO_FTRACE_REGS_CAST */
> > +
> >  /*
> >   * When true, the ftrace_regs_{get,set}_*() functions may be used on fregs.
> >   * Note: this can be true even when ftrace_get_regs() cannot provide a pt_regs.
> >
> >


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>