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

Petr Mladek
2018-05-24T11:40:24Z

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@google.com>
> ---
> 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