[PATCH v2] entry: Respect changes to system call number at sys_enter tracepoint

["André Rösti" <an.roesti@gmail.com>]

When a probe  is registered at the trace_sys_enter() tracepoint, and that
probe changes the system call number, the old system call still gets
executed.  This worked correctly until commit b6ec41346103 ("core/entry:
Report syscall correctly for trace and audit"), which removed the
re-evaluation of the syscall number after the trace point.

Restore the original semantics by re-evaluating the system call number
after trace_sys_enter(). 

The performance impact of this re-evaluation is minimal because it only
takes place when a trace point is active, and compared to the actual trace
point overhead the read from a cache hot variable is negligible.

Fixes: b6ec41346103 ("core/entry: Report syscall correctly for trace and audit")
Signed-off-by: André Rösti <an.roesti@gmail.com>

---
v1 -> v2: Clarify comment; improve commit message

@Thomas Gleixner: Thank you for your very quick reply and helpful
comments. I copied and slightly reworded your suggestions for the commit
message which were much more concise and clear. 
---
 kernel/entry/common.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 88cb3c88aaa5..90843cc38588 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -57,8 +57,14 @@ long syscall_trace_enter(struct pt_regs *regs, long syscall,
 	/* Either of the above might have changed the syscall number */
 	syscall = syscall_get_nr(current, regs);
 
-	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT))
+	if (unlikely(work & SYSCALL_WORK_SYSCALL_TRACEPOINT)) {
 		trace_sys_enter(regs, syscall);
+		/*
+		 * Probes or BPF hooks in the tracepoint may have changed the
+		 * system call number as well.
+		 */
+		syscall = syscall_get_nr(current, regs);
+	}
 
 	syscall_enter_audit(regs, syscall);
 

base-commit: 221a164035fd8b554a44bd7c4bf8e7715a497561
-- 
2.34.1