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.
> ---
> 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.
> 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
[...]
> 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
[...]
> 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.
> +" add sp, sp, #48\n"
> +" ret x9\n"
> +" .size my_tramp, .-my_tramp\n"
> +" .popsection\n"
> +);
> +
> +#endif /* CONFIG_ARM64 */
> +
> static struct ftrace_ops direct;
>
> static int __init ftrace_direct_init(void)
> diff --git a/samples/ftrace/ftrace-direct.c b/samples/ftrace/ftrace-direct.c
> index d81a9473b585..e5312f9c15d3 100644
> --- a/samples/ftrace/ftrace-direct.c
> +++ b/samples/ftrace/ftrace-direct.c
> @@ -3,7 +3,9 @@
>
> #include <linux/sched.h> /* for wake_up_process() */
> #include <linux/ftrace.h>
> +#ifndef CONFIG_ARM64
> #include <asm/asm-offsets.h>
> +#endif
>
> extern void my_direct_func(struct task_struct *p);
>
> @@ -63,6 +65,28 @@ 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"
> +);
This looks fine. Since my_direct_func() is attached to wake_up_process() and
expects its single argument, saving/restoring x0 is sufficient and we don't
need any additional register shuffling.
Thanks,
Mark.
