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SubjectRe: [PATCH] [GIT PULL] tracing: allow to change permissions for text with dynamic ftrace enabled
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On Tue, 2009-10-27 at 14:26 -0800, Suresh Siddha wrote:
> On Tue, 2009-10-27 at 11:33 -0700, Steven Rostedt wrote:
> > On Tue, 2009-10-27 at 11:20 -0800, Suresh Siddha wrote:
> >
> > > Steven, Is it possible for dynamic ftrace to use the kernel identity
> > > mapping instead of the kernel text mapping for converting the tracing
> > > calls to nops etc.
> >
> > Not sure what you mean by "kernel identity" mapping.
>
> 64bit has the kernel text pages mapped at two locations. kernel identity
> mapping (__PAGE_OFFSET) and kernel image/text mapping
> (__START_KERNEL_map).

I didn't realize that x86_64 mapped it two different ways.

>
> DEBUG_RODATA patch was trying to preserve large page mapping (for perf
> reasons) for kernel text. We can use the identity mapping for modifying
> the kernel text.
>
> This patch seems to fix.
>
> diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> index 5a1b975..7e1799b 100644
> --- a/arch/x86/kernel/ftrace.c
> +++ b/arch/x86/kernel/ftrace.c
> @@ -215,6 +215,11 @@ do_ftrace_mod_code(unsigned long ip, void *new_code)
> }
>
>
> +static inline int
> +within(unsigned long addr, unsigned long start, unsigned long end)
> +{
> + return addr >= start && addr < end;
> +}
>
>
> static unsigned char ftrace_nop[MCOUNT_INSN_SIZE];
> @@ -248,6 +253,14 @@ ftrace_modify_code(unsigned long ip, unsigned char *old_code,
> if (memcmp(replaced, old_code, MCOUNT_INSN_SIZE) != 0)
> return -EINVAL;
>
> + /*
> + * On x86_64, we use the kernel identity mapping instead of the
> + * kernel text mapping to modify the kernel text. This is a nop
> + * for 32bit kernels.
> + */

Is it really a nop on 32bit? Does it just turn into ip = ip?


> + if (within(ip, (unsigned long)_text, (unsigned long)_etext))
> + ip = (unsigned long)__va(__pa(ip));
> +
> /* replace the text with the new text */
> if (do_ftrace_mod_code(ip, new_code))
> return -EPERM;

I'll test it out, and if it does work, you can write up a formal patch
and remove the !define that I added.

Thanks,

-- Steve




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