Hi yet again Alex!
You didn't confirm so I was wondering if you received my message!
Let me know if possible.
-- fvalasiad --
On 27/2/23 21:19, Φώτης Βαλασιάδης wrote:
Hi Alex!
Is that good enough?
-- fvalasiad --
On 27/2/23 21:17, 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@xxxxxxxxx>
Signed-off-by: Dmitry V. Levin <ldv@xxxxxxxxx>
Signed-off-by: Fotios Valasiadis <fvalasiad@xxxxxxxxx>
Acked-by: Nate Eldredge <nate@xxxxxxxxxxxxxxxxxxxx>
Cc: Elvira Khabirova <lineprinter0@xxxxxxxxx>
Signed-off-by: Alejandro Colomar <alx@xxxxxxxxxx>
---
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
