Re: [PATCH] ptrace.2: Add details about usage of PTRACE_GET_SYSCALL_INFO

[Alejandro Colomar <alx.manpages@gmail.com>]

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Hi Fotios,

On 2/27/23 20:19, Φώτης Βαλασιάδης wrote:
> Hi Alex!
> 
> Is that good enough?
> 

Yep.  Patch applied.  Thanks!

Alex

On 2/27/23 20:17, Fotios Valasiadis wrote:
> Document the role of PTRACE_O_TRACESYSGOOD option in connection with
> PTRACE_GET_SYSCALL_INFO.
> 
> Came upon this after writing a test program using PTRACE_GET_SYSCALL_INFO.
> After failing to find what's wrong I posted a StackOverflow question
> which you can find right here:
> <https://stackoverflow.com/questions/72410182/ptrace-get-syscall-info-always-returns-info-op-as-ptrace-syscall-info-none>
> 
> Nate Eldredge found out what happens by looking into the kernel's source
> code, here is a link to the relevant part
> <https://github.com/torvalds/linux/blob/8291eaafed36f575f23951f3ce18407f480e9ecf/kernel/ptrace.c#L1018>
> 
> In the code it can be seen that in case of system call entry or
> exit stops, the union is filled if and only if the signal matches
> "SIGTRAP | 0x80", a signal which is only sent if the PTRACE_O_TRACESYSGOOD
> option is set.  You can read about that in the PTRACE_O_TRACESYSGOOD
> section of ptrace(2)'s manual.
> 
> Complements: fc91449cb "ptrace.2: Document PTRACE_GET_SYSCALL_INFO"
> Cowritten-by: Dmitry V. Levin <ldv@strace.io>
> Signed-off-by: Dmitry V. Levin <ldv@strace.io>
> Signed-off-by: Fotios Valasiadis <fvalasiad@gmail.com>
> Acked-by: Nate Eldredge <nate@thatsmathematics.com>
> Cc: Elvira Khabirova <lineprinter0@gmail.com>
> Signed-off-by: Alejandro Colomar <alx@kernel.org>
> ---
>  man2/ptrace.2 | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/man2/ptrace.2 b/man2/ptrace.2
> index 55d9fd36d..9737b3825 100644
> --- a/man2/ptrace.2
> +++ b/man2/ptrace.2
> @@ -1111,6 +1111,15 @@ stop.
>  .B PTRACE_SYSCALL_INFO_NONE
>  No component of the union contains relevant information.
>  .RE
> +.IP
> +In case of system call entry or exit stops,
> +the data returned by
> +.B PTRACE_GET_SYSCALL_INFO
> +is limited to type
> +.B PTRACE_SYSCALL_INFO_NONE
> +unless
> +.B PTRACE_O_TRACESYSGOOD
> +option is set before the corresponding system call stop has occurred.
>  .\"
>  .SS Death under ptrace
>  When a (possibly multithreaded) process receives a killing signal

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