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

[Steven Rostedt <rostedt@xxxxxxxxxxx>]

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.

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

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


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