Re: [Xen-devel] [PATCH v3 21/27] x86/ftrace: Adapt function tracing for PIE support

[Thomas Garnier <thgarnie@xxxxxxxxxx>]

On Thu, May 24, 2018 at 1:41 PM Thomas Garnier <thgarnie@xxxxxxxxxx> wrote:


> On Thu, May 24, 2018 at 1:16 PM Steven Rostedt <rostedt@xxxxxxxxxxx>
wrote:

> > On Thu, 24 May 2018 13:40:24 +0200
> > Petr Mladek <pmladek@xxxxxxxx> wrote:

> > > On Wed 2018-05-23 12:54:15, Thomas Garnier wrote:
> > > > When using -fPIE/PIC with function tracing, the compiler generates a
> > > > call through the GOT (call *__fentry__@GOTPCREL). This instruction
> > > > takes 6 bytes instead of 5 on the usual relative call.
> > > >
> > > > If PIE is enabled, replace the 6th byte of the GOT call by a 1-byte
> nop
> > > > so ftrace can handle the previous 5-bytes as before.
> > > >
> > > > Position Independent Executable (PIE) support will allow to extended
> the
> > > > KASLR randomization range below the -2G memory limit.
> > > >
> > > > Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > > > ---
> > > >  arch/x86/include/asm/ftrace.h   |  6 +++--
> > > >  arch/x86/include/asm/sections.h |  4 ++++
> > > >  arch/x86/kernel/ftrace.c        | 42
> +++++++++++++++++++++++++++++++--
> > > >  3 files changed, 48 insertions(+), 4 deletions(-)
> > > >
> > > > diff --git a/arch/x86/include/asm/ftrace.h
> b/arch/x86/include/asm/ftrace.h
> > > > index c18ed65287d5..8f2decce38d8 100644
> > > > --- a/arch/x86/include/asm/ftrace.h
> > > > +++ b/arch/x86/include/asm/ftrace.h
> > > > @@ -25,9 +25,11 @@ extern void __fentry__(void);
> > > >  static inline unsigned long ftrace_call_adjust(unsigned long addr)
> > > >  {
> > > >     /*
> > > > -    * addr is the address of the mcount call instruction.
> > > > -    * recordmcount does the necessary offset calculation.
> > > > +    * addr is the address of the mcount call instruction. PIE has
> always a
> > > > +    * byte added to the start of the function.
> > > >      */
> > > > +   if (IS_ENABLED(CONFIG_X86_PIE))
> > > > +           addr -= 1;
> > >
> > > This seems to modify the address even for modules that are _not_
> compiled with
> > > PIE, see below.

> > Can one load a module not compiled for PIE in a kernel with PIE?

> > >
> > > >     return addr;
> > > >  }
> > > >
> > > > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
> > > > index 01ebcb6f263e..73b3c30cb7a3 100644
> > > > --- a/arch/x86/kernel/ftrace.c
> > > > +++ b/arch/x86/kernel/ftrace.c
> > > > @@ -135,6 +135,44 @@ ftrace_modify_code_direct(unsigned long ip,
> unsigned const char *old_code,
> > > >     return 0;
> > > >  }
> > > >
> > > > +/* Bytes before call GOT offset */
> > > > +const unsigned char got_call_preinsn[] = { 0xff, 0x15 };
> > > > +
> > > > +static int
> > > > +ftrace_modify_initial_code(unsigned long ip, unsigned const char
> *old_code,
> > > > +                      unsigned const char *new_code)
> > > > +{
> > > > +   unsigned char replaced[MCOUNT_INSN_SIZE + 1];
> > > > +
> > > > +   ftrace_expected = old_code;
> > > > +
> > > > +   /*
> > > > +    * If PIE is not enabled or no GOT call was found, default to
the
> > > > +    * original approach to code modification.
> > > > +    */
> > > > +   if (!IS_ENABLED(CONFIG_X86_PIE) ||
> > > > +       probe_kernel_read(replaced, (void *)ip, sizeof(replaced)) ||
> > > > +       memcmp(replaced, got_call_preinsn,
sizeof(got_call_preinsn)))
> > > > +           return ftrace_modify_code_direct(ip, old_code,
new_code);
> > >
> > > And this looks like an attempt to handle modules compiled without
> > > PIE. Does it works with the right ip in that case?

> > I'm guessing the || is for the "or no GOT call was found", but it
> > doesn't explain why no GOT would be found.

> Yes, maybe I could have made it work by using text_ip_addr earlier.


> > >
> > > I wonder if a better solution would be to update
> > > scripts/recordmcount.c to store the incremented location into the
> module.

> I will look into it.

Found a way to properly change the __mcount_loc using the preprocessing
(removing the need for -1 on the addr). It will be part of the next version.

Thanks for the feedback.



> > If recordmcount.c can handle this, then I think that's the preferred
> > approach. Thanks!

> > -- Steve

> > >
> > > IMPORTANT: I have only vague picture about how this all works. It is
> > > possible that I am completely wrong. The code might be correct,
> > > especially if you tested this situation.
> > >
> > > Best Regards,
> > > Petr



> --
> Thomas



-- 
Thomas

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