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

[Petr Mladek <pmladek@xxxxxxxx>]

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.

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

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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