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

[Andrii Nakryiko]

On Wed, Aug 23, 2023 at 8:16 AM Masami Hiramatsu (Google)
<mhiramat@xxxxxxxxxx> wrote:
>
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Acked-by: Florent Revest <revest@xxxxxxxxxxxx>
> ---
>  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?

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.


> +       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.
>
>