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

["Dmitry V. Levin" <ldv@strace.io>]

On Fri, Feb 24, 2023 at 06:31:42PM +0200, Fotios Valasiadis wrote:
> From: Fotios Valasiadis <fvalasiad@csd.auth.gr>
> 
> Fixed incomplete doc. PTRACE_GET_SYSCALL_INFO's description wouldn't inform the user that they need to enable PTRACE_O_TRACESYSGOOD to get detailed information from said operation.
> 
> Came upon this bug after writing a test program using PTRACE_O_TRACESYSGOOD. 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's wrong 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 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.
> 
> I am unaware if Nate wants to be included as a co-author of this commit, nor am I aware of any way to contact him.

Cc'ed Nate just in case.

> Signed-off-by: Fotios Valasiadis <fvalasiad@gmail.com>
> ---
>  man2/ptrace.2 | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/man2/ptrace.2 b/man2/ptrace.2
> index 55d9fd36d..b437b4f44 100644
> --- a/man2/ptrace.2
> +++ b/man2/ptrace.2
> @@ -1033,6 +1033,11 @@ If the size of the data to be written by the kernel exceeds the size
>  specified by the
>  .I addr
>  argument, the output data is truncated.
> +To obtain complete information during system call entry or exit stops,
> +you should enable the
> +.B PTRACE_O_TRACESYSGOOD
> +option using
> +.B PTRACE_SETOPTIONS.
>  .IP
>  The
>  .I ptrace_syscall_info

I suggest the following wording:

diff --git a/man2/ptrace.2 b/man2/ptrace.2
index 55d9fd36d..496d8fae0 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
+Note that 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 ptrace stop has occurred.
 .\"
 .SS Death under ptrace
 When a (possibly multithreaded) process receives a killing signal

As written in the manual page, using the PTRACE_O_TRACESYSGOOD option is
the recommended method to distinguish syscall-stops from other kinds of
ptrace-stops, since it is reliable and does not incur a performance
penalty.  In other words, if you use ptrace(2) at all, please consider
using PTRACE_O_TRACESYSGOOD option.


-- 
ldv