Re: [PATCH v13 bpf-next 1/1] rethook: x86: Add rethook x86 implementation

[Masami Hiramatsu]

On Wed, 23 Mar 2022 09:05:26 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Wed, Mar 23, 2022 at 11:34:59AM +0900, Masami Hiramatsu wrote:
> > Add rethook for x86 implementation. Most of the code has been copied from
> > kretprobes on x86.
> 
> Right; as said, I'm really unhappy with growing a carbon copy of this
> stuff instead of sharing. Can we *please* keep it a single instance?

OK, then let me update the kprobe side too.

> Them being basically indentical, it should be trivial to have
> CONFIG_KPROBE_ON_RETHOOK (or somesuch) and just share this.

Yes, ideally it should use CONFIG_HAVE_RETHOOK since the rethook arch port
must be a copy of the kretprobe implementation. But for safety, I think
having CONFIG_KPROBE_ON_RETHOOK is a good idea until replacing all kretprobe
implementations.

> 
> Also, what's rethook for anyway?

Rethook is a feature which hooks the function return. Most of the
logic came from the kretprobe. Simply to say, 'kretprobe - kprobe' is 
the rethook :)

Thank you,

> 
> > diff --git a/arch/x86/kernel/kprobes/common.h b/arch/x86/kernel/kprobes/common.h
> > index 7d3a2e2daf01..c993521d4933 100644
> > --- a/arch/x86/kernel/kprobes/common.h
> > +++ b/arch/x86/kernel/kprobes/common.h
> > @@ -6,6 +6,7 @@
> >  
> >  #include <asm/asm.h>
> >  #include <asm/frame.h>
> > +#include <asm/insn.h>
> >  
> >  #ifdef CONFIG_X86_64
> >  
> > diff --git a/arch/x86/kernel/rethook.c b/arch/x86/kernel/rethook.c
> > new file mode 100644
> > index 000000000000..3e916361c33b
> > --- /dev/null
> > +++ b/arch/x86/kernel/rethook.c
> > @@ -0,0 +1,121 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * x86 implementation of rethook. Mostly copied from arch/x86/kernel/kprobes/core.c.
> > + */
> > +#include <linux/bug.h>
> > +#include <linux/rethook.h>
> > +#include <linux/kprobes.h>
> > +#include <linux/objtool.h>
> > +
> > +#include "kprobes/common.h"
> > +
> > +__visible void arch_rethook_trampoline_callback(struct pt_regs *regs);
> > +
> > +/*
> > + * When a target function returns, this code saves registers and calls
> > + * arch_rethook_trampoline_callback(), which calls the rethook handler.
> > + */
> > +asm(
> > +	".text\n"
> > +	".global arch_rethook_trampoline\n"
> > +	".type arch_rethook_trampoline, @function\n"
> > +	"arch_rethook_trampoline:\n"
> > +#ifdef CONFIG_X86_64
> > +	ANNOTATE_NOENDBR	/* This is only jumped from ret instruction */
> > +	/* Push a fake return address to tell the unwinder it's a kretprobe. */
> > +	"	pushq $arch_rethook_trampoline\n"
> > +	UNWIND_HINT_FUNC
> 	"	pushq $" __stringify(__KERNEL_DS) "\n" /* %ss */
> 	/* Save the 'sp - 16', this will be fixed later. */
> > +	"	pushq %rsp\n"
> > +	"	pushfq\n"
> > +	SAVE_REGS_STRING
> > +	"	movq %rsp, %rdi\n"
> > +	"	call arch_rethook_trampoline_callback\n"
> > +	RESTORE_REGS_STRING
> 	/* In the callback function, 'regs->flags' is copied to 'regs->ss'. */
> 
> 	this comment could do with a 'why' though... Because neither
> 	this nor the one in the handler really explains why it is
> 	important to have popf last
> 
> 	"	addq $16, %rsp\n"
> > +	"	popfq\n"
> > +#else
> 
> same for i386:
> 
> > +	/* Push a fake return address to tell the unwinder it's a kretprobe. */
> > +	"	pushl $arch_rethook_trampoline\n"
> > +	UNWIND_HINT_FUNC
> 	/* Save the 'sp - 8', this will be fixed later. */
> 	"	pushl %ss\n"
> > +	"	pushl %esp\n"
> > +	"	pushfl\n"
> > +	SAVE_REGS_STRING
> > +	"	movl %esp, %eax\n"
> > +	"	call arch_rethook_trampoline_callback\n"
> > +	RESTORE_REGS_STRING
> 	/* In the callback function, 'regs->flags' is copied to 'regs->ss'. */
> 	"	addl $8, %esp\n"
> > +	"	popfl\n"
> > +#endif
> > +	ASM_RET
> > +	".size arch_rethook_trampoline, .-arch_rethook_trampoline\n"
> > +);
> > +NOKPROBE_SYMBOL(arch_rethook_trampoline);
> > +
> > +/*
> > + * Called from arch_rethook_trampoline
> > + */
> > +__used __visible void arch_rethook_trampoline_callback(struct pt_regs *regs)
> > +{
> > +	unsigned long *frame_pointer;
> > +
> > +	/* fixup registers */
> > +	regs->cs = __KERNEL_CS;
> > +#ifdef CONFIG_X86_32
> > +	regs->gs = 0;
> > +#endif
> > +	regs->ip = (unsigned long)&arch_rethook_trampoline;
> > +	regs->orig_ax = ~0UL;
> 	regs->sp += 2*sizeof(long);
> > +	frame_pointer = &regs->sp + 1;
> > +
> > +	/*
> > +	 * The return address at 'frame_pointer' is recovered by the
> > +	 * arch_rethook_fixup_return() which called from this
> > +	 * rethook_trampoline_handler().
> > +	 */
> > +	rethook_trampoline_handler(regs, (unsigned long)frame_pointer);
> > +
> > +	/*
> > +	 * Copy FLAGS to 'pt_regs::sp' so that arch_rethook_trapmoline()
> > +	 * can do RET right after POPF.
> > +	 */
> 	regs->ss = regs->flags;
> > +}
> > +NOKPROBE_SYMBOL(arch_rethook_trampoline_callback);


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>