Re: [PATCH v5 3/4] arm64: ftrace: Add direct call trampoline samples support

[Florent Revest]

On Mon, Apr 3, 2023 at 7:26 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Mon, Apr 03, 2023 at 01:35:51PM +0200, Florent Revest wrote:
> > The ftrace samples need per-architecture trampoline implementations
> > to save and restore argument registers around the calls to
> > my_direct_func* and to restore polluted registers (eg: x30).
> >
> > These samples also include <asm/asm-offsets.h> which, on arm64, is not
> > necessary and redefines previously defined macros (resulting in
> > warnings) so these includes are guarded by !CONFIG_ARM64.
> >
> > Signed-off-by: Florent Revest <revest@xxxxxxxxxxxx>
>
> Overall this looks pretty good!
>
> I spotted a few bugs below while testing, and I've suggested some fixups below.
>
> w.r.t. the asm-offsets include guards. I took a look at fixing arm64's
> asm-offsets.c to not be problematic, but it requires some invasive refactoring,
> so I'd like to clean that up as a separate series. I don't think that should
> block this series, and I think that the include guards are fine for now.

Sounds great! Thank you Mark :)

> > ---
> >  arch/arm64/Kconfig                          |  2 ++
> >  samples/ftrace/ftrace-direct-modify.c       | 34 ++++++++++++++++++
> >  samples/ftrace/ftrace-direct-multi-modify.c | 38 +++++++++++++++++++++
> >  samples/ftrace/ftrace-direct-multi.c        | 23 +++++++++++++
> >  samples/ftrace/ftrace-direct-too.c          | 26 ++++++++++++++
> >  samples/ftrace/ftrace-direct.c              | 24 +++++++++++++
> >  6 files changed, 147 insertions(+)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index f3503d0cc1b8..c2bf28099abd 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -194,6 +194,8 @@ config ARM64
> >                   !CC_OPTIMIZE_FOR_SIZE)
> >       select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
> >               if DYNAMIC_FTRACE_WITH_ARGS
> > +     select HAVE_SAMPLE_FTRACE_DIRECT
> > +     select HAVE_SAMPLE_FTRACE_DIRECT_MULTI
> >       select HAVE_EFFICIENT_UNALIGNED_ACCESS
> >       select HAVE_FAST_GUP
> >       select HAVE_FTRACE_MCOUNT_RECORD
> > diff --git a/samples/ftrace/ftrace-direct-modify.c b/samples/ftrace/ftrace-direct-modify.c
> > index 25fba66f61c0..98d1b7385f08 100644
> > --- a/samples/ftrace/ftrace-direct-modify.c
> > +++ b/samples/ftrace/ftrace-direct-modify.c
> > @@ -2,7 +2,9 @@
> >  #include <linux/module.h>
> >  #include <linux/kthread.h>
> >  #include <linux/ftrace.h>
> > +#ifndef CONFIG_ARM64
> >  #include <asm/asm-offsets.h>
> > +#endif
> >
> >  extern void my_direct_func1(void);
> >  extern void my_direct_func2(void);
> > @@ -96,6 +98,38 @@ asm (
> >
> >  #endif /* CONFIG_S390 */
> >
> > +#ifdef CONFIG_ARM64
> > +
> > +asm (
> > +"    .pushsection    .text, \"ax\", @progbits\n"
> > +"    .type           my_tramp1, @function\n"
> > +"    .globl          my_tramp1\n"
> > +"   my_tramp1:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #16\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    bl      my_direct_func1\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    add     sp, sp, #16\n"
> > +"    ret     x9\n"
> > +"    .size           my_tramp1, .-my_tramp1\n"
> > +
> > +"    .type           my_tramp2, @function\n"
> > +"    .globl          my_tramp2\n"
> > +"   my_tramp2:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #16\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    bl      my_direct_func2\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    add     sp, sp, #16\n"
> > +"    ret     x9\n"
> > +"    .size           my_tramp2, .-my_tramp2\n"
> > +"    .popsection\n"
> > +);
> > +
> > +#endif /* CONFIG_ARM64 */
>
> These looks functionally correct, given they'll only be attached to schedule()
> and the direct funcs take no arguments, so there's no arguments to save/restore
> and nothing to shuffle.
>
> As an aside, I believe we'll need to rework the sequences when we add support
> for RELIABLE_STACKTRACE so that the unwinder can reliably acquire the address
> of the instrumented function and its caller, but I think for now it's
> preferable to keep this simple and I'm happy to make that a problem for future
> me.

Ah, interesting. I'd be happy to help when that time comes! :)

> > diff --git a/samples/ftrace/ftrace-direct-multi-modify.c b/samples/ftrace/ftrace-direct-multi-modify.c
> > index f72623899602..e39108eb085d 100644
> > --- a/samples/ftrace/ftrace-direct-multi-modify.c
> > +++ b/samples/ftrace/ftrace-direct-multi-modify.c
> > @@ -2,7 +2,9 @@
> >  #include <linux/module.h>
> >  #include <linux/kthread.h>
> >  #include <linux/ftrace.h>
> > +#ifndef CONFIG_ARM64
> >  #include <asm/asm-offsets.h>
> > +#endif
> >
> >  extern void my_direct_func1(unsigned long ip);
> >  extern void my_direct_func2(unsigned long ip);
> > @@ -103,6 +105,42 @@ asm (
> >
> >  #endif /* CONFIG_S390 */
> >
> > +#ifdef CONFIG_ARM64
> > +
> > +asm (
> > +"    .pushsection    .text, \"ax\", @progbits\n"
> > +"    .type           my_tramp1, @function\n"
> > +"    .globl          my_tramp1\n"
> > +"   my_tramp1:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #32\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    str     x0, [sp, #16]\n"
> > +"    bl      my_direct_func1\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    ldr     x0, [sp, #16]\n"
> > +"    add     sp, sp, #32\n"
> > +"    ret     x9\n"
> > +"    .size           my_tramp1, .-my_tramp1\n"
> > +
> > +"    .type           my_tramp2, @function\n"
> > +"    .globl          my_tramp2\n"
> > +"   my_tramp2:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #32\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    str     x0, [sp, #16]\n"
> > +"    bl      my_direct_func2\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    ldr     x0, [sp, #16]\n"
> > +"    add     sp, sp, #32\n"
> > +"    ret     x9\n"
> > +"    .size           my_tramp2, .-my_tramp2\n"
> > +"    .popsection\n"
> > +);
> > +
> > +#endif /* CONFIG_ARM64 */
>
> For both of these trampolines we need to pass the trampoline's return address
> (i.e. where we'll return to in the instrumented function) as the 'ip' argument
> to my_direct_func{1,2}().
>
> In both cases, just before the 'bl my_direct_func{1,2}' we'll need to add:
>
>         mov     x0, x30

Oopsie, yes! Very good catch!

> > diff --git a/samples/ftrace/ftrace-direct-multi.c b/samples/ftrace/ftrace-direct-multi.c
> > index 1547c2c6be02..5a395d2d2e07 100644
> > --- a/samples/ftrace/ftrace-direct-multi.c
> > +++ b/samples/ftrace/ftrace-direct-multi.c
> > @@ -4,7 +4,9 @@
> >  #include <linux/mm.h> /* for handle_mm_fault() */
> >  #include <linux/ftrace.h>
> >  #include <linux/sched/stat.h>
> > +#ifndef CONFIG_ARM64
> >  #include <asm/asm-offsets.h>
> > +#endif
> >
> >  extern void my_direct_func(unsigned long ip);
> >
> > @@ -66,6 +68,27 @@ asm (
> >
> >  #endif /* CONFIG_S390 */
> >
> > +#ifdef CONFIG_ARM64
> > +
> > +asm (
> > +"    .pushsection    .text, \"ax\", @progbits\n"
> > +"    .type           my_tramp, @function\n"
> > +"    .globl          my_tramp\n"
> > +"   my_tramp:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #32\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    str     x0, [sp, #16]\n"
> > +"    bl      my_direct_func\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    ldr     x0, [sp, #16]\n"
> > +"    add     sp, sp, #32\n"
> > +"    ret     x9\n"
> > +"    .size           my_tramp, .-my_tramp\n"
> > +"    .popsection\n"
> > +);
> > +
> > +#endif /* CONFIG_ARM64 */
> >  static struct ftrace_ops direct;
>
> As with ftrace-direct-multi-modify.c, we need to pass the return address of the
> trampoline as the 'ip' argument to my_direct_func1(), so just before the 'bl
> my_direct_func' we'll need to add:
>
>         mov     x0, x30

Will do :)

> > diff --git a/samples/ftrace/ftrace-direct-too.c b/samples/ftrace/ftrace-direct-too.c
> > index f28e7b99840f..6e93c45fea86 100644
> > --- a/samples/ftrace/ftrace-direct-too.c
> > +++ b/samples/ftrace/ftrace-direct-too.c
> > @@ -3,7 +3,9 @@
> >
> >  #include <linux/mm.h> /* for handle_mm_fault() */
> >  #include <linux/ftrace.h>
> > +#ifndef CONFIG_ARM64
> >  #include <asm/asm-offsets.h>
> > +#endif
> >
> >  extern void my_direct_func(struct vm_area_struct *vma,
> >                          unsigned long address, unsigned int flags);
>
> This gets attached to handle_mm_fault(), whose prototype is currently:
>
> vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
>                            unsigned int flags, struct pt_regs *regs)
>
> i.e. it has 4 arguments, in x0 to x3.
>
> > @@ -70,6 +72,30 @@ asm (
> >
> >  #endif /* CONFIG_S390 */
> >
> > +#ifdef CONFIG_ARM64
> > +
> > +asm (
> > +"    .pushsection    .text, \"ax\", @progbits\n"
> > +"    .type           my_tramp, @function\n"
> > +"    .globl          my_tramp\n"
> > +"   my_tramp:"
> > +"    bti     c\n"
> > +"    sub     sp, sp, #48\n"
> > +"    stp     x9, x30, [sp]\n"
> > +"    stp     x0, x1, [sp, #16]\n"
> > +"    str     x2, [sp, #32]\n"
> > +"    bl      my_direct_func\n"
> > +"    ldp     x30, x9, [sp]\n"
> > +"    ldp     x0, x1, [sp, #16]\n"
> > +"    ldr     x2, [sp, #32]\n"
>
> So here we need to save+restore x3 also.
>
> We already have the space reserved, so that should just be a matter of using
> stp/ldp for x2 and x3.

That's also a very good catch. It looks like it's an issue for the
sample trampoline on x86_64 as well, I'll fix it too in a separate
patch as part of v6. (if i understand s390 asm correctly, the stmg
already saves all arg registers in one instruction so s390 doesn't
need to change)