bpf: Lock bpf_trace_printk's tmp buf before it is written to

bpf_trace_printk uses a shared static buffer to hold strings before they
are printed. A recent refactoring moved the locking of that buffer after
it gets filled by mistake.

Fixes: d9c9e4db ("bpf: Factorize bpf_trace_printk and bpf_seq_printf")
Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Florent Revest <revest@chromium.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210427112958.773132-1-revest@chromium.org

kernel/trace/bpf_trace.c

@@ -391,13 +391,13 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
 	if (ret < 0)
 		return ret;
 
+	raw_spin_lock_irqsave(&trace_printk_lock, flags);
 	ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
 		BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
 	/* snprintf() will not append null for zero-length strings */
 	if (ret == 0)
 		buf[0] = '\0';
 
-	raw_spin_lock_irqsave(&trace_printk_lock, flags);
 	trace_bpf_trace_printk(buf);
 	raw_spin_unlock_irqrestore(&trace_printk_lock, flags);