Re: [PATCH v2 09/13] powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS

Christophe Leroy [off-list ref] writes:

Le 14/02/2022 à 16:25, Naveen N. Rao a écrit :

quoted

Christophe Leroy wrote:

quoted

Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS. It accelerates the call
of livepatching.

Also note that powerpc being the last one to convert to
CONFIG_DYNAMIC_FTRACE_WITH_ARGS, it will now be possible to remove
klp_arch_set_pc() on all architectures.

Signed-off-by: Christophe Leroy <redacted>
---
 arch/powerpc/Kconfig                 |  1 +
 arch/powerpc/include/asm/ftrace.h    | 17 +++++++++++++++++
 arch/powerpc/include/asm/livepatch.h |  4 +---
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index cdac2115eb00..e2b1792b2aae 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig

@@ -210,6 +210,7 @@ config PPC

     select HAVE_DEBUG_KMEMLEAK
     select HAVE_DEBUG_STACKOVERFLOW
     select HAVE_DYNAMIC_FTRACE
+    select HAVE_DYNAMIC_FTRACE_WITH_ARGS    if MPROFILE_KERNEL || PPC32
     select HAVE_DYNAMIC_FTRACE_WITH_REGS    if MPROFILE_KERNEL || PPC32
     select HAVE_EBPF_JIT
     select HAVE_EFFICIENT_UNALIGNED_ACCESS    if !(CPU_LITTLE_ENDIAN 
&& POWER7_CPU)

diff --git a/arch/powerpc/include/asm/ftrace.h 

b/arch/powerpc/include/asm/ftrace.h
index b3f6184f77ea..45c3d6f11daa 100644

--- a/arch/powerpc/include/asm/ftrace.h
+++ b/arch/powerpc/include/asm/ftrace.h

@@ -22,6 +22,23 @@ static inline unsigned long 

ftrace_call_adjust(unsigned long addr)
 struct dyn_arch_ftrace {
     struct module *mod;
 };
+
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_ARGS
+struct ftrace_regs {
+    struct pt_regs regs;
+};
+
+static __always_inline struct pt_regs *arch_ftrace_get_regs(struct 
ftrace_regs *fregs)
+{
+    return &fregs->regs;
+}

I think this is wrong. We need to differentiate between ftrace_caller() 
and ftrace_regs_caller() here, and only return pt_regs if coming in 
through ftrace_regs_caller() (i.e., FL_SAVE_REGS is set).

Not sure I follow you.

This is based on 5740a7c71ab6 ("s390/ftrace: add 
HAVE_DYNAMIC_FTRACE_WITH_ARGS support")

It's all the point of HAVE_DYNAMIC_FTRACE_WITH_ARGS, have the regs also 
with ftrace_caller().

Sure you only have the params, but that's the same on s390, so what did 
I miss ?

I already have this series in next, I can pull it out, but I'd rather
not.

I'll leave it in for now, hopefully you two can agree overnight my time
whether this is a big problem or something we can fix with a fixup
patch.

quoted

quoted

+static __always_inline void ftrace_instruction_pointer_set(struct 
ftrace_regs *fregs,
+                               unsigned long ip)
+{
+    regs_set_return_ip(&fregs->regs, ip);

Should we use that helper here? regs_set_return_ip() also updates some 
other state related to taking interrupts and I don't think it makes 
sense for use with ftrace.

Today we have:

	static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned 
long ip)
	{
		struct pt_regs *regs = ftrace_get_regs(fregs);

		regs_set_return_ip(regs, ip);
	}


Which like x86 and s390 becomes:

	static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned 
long ip)
	{
		ftrace_instruction_pointer_set(fregs, ip);
	}



That's the reason why I've been using regs_set_return_ip(). Do you think 
it was wrong to use regs_set_return_ip() in klp_arch_set_pc() ?

That was added by 59dc5bfca0cb ("powerpc/64s: avoid reloading (H)SRR 
registers if they are still valid")

It's not wrong, but I think it's unnecessary. We need to use
regs_set_return_ip() if we're changing the regs->ip of an interrupt
frame, so that the interrupt return code will reload it.

But AIUI in this case we're not doing that, we're changing the regs->ip
of a pt_regs provided by ftrace, which shouldn't ever be an interrupt
frame.

So it's not a bug to use regs_set_return_ip(), but it is unncessary and
means we'll reload the interrupt state unnecessarily on the next
interrupt return.

cheers