On Sat, 25 Feb 2023, Alex Colomar wrote:
On 2/24/23 17:31, Fotios Valasiadis wrote:
From: Fotios Valasiadis <fvalasiad@xxxxxxxxxxx>
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.
Hello Fotios,
I found his address in a kernel commit:
Nate Eldredge <nate@xxxxxxxxxxxxxxxxxxxx>
I'm CCing him in case he wants to review.
Hi everyone, I think it's a good clear explanation. Thanks for taking the
time to write the patch! No need to include me as coauthor.
Thanks for the patch!
Alex
Signed-off-by: Fotios Valasiadis <fvalasiad@xxxxxxxxx>
---
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
--
<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
--
Nate Eldredge
nate@xxxxxxxxxxxxxxxxxxxx
