Re: [PATCH RFC v4 net-next 17/26] tracing: allow eBPF programs to be attached to events

On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov [off-list ref] wrote:
[...]

+/* For tracing filters save first six arguments of tracepoint events.
+ * On 64-bit architectures argN fields will match one to one to arguments passed
+ * to tracepoint events.
+ * On 32-bit architectures u64 arguments to events will be seen into two
+ * consecutive argN, argN+1 fields. Pointers, u32, u16, u8, bool types will
+ * match one to one
+ */
+struct bpf_context {
+       unsigned long arg1;
+       unsigned long arg2;
+       unsigned long arg3;
+       unsigned long arg4;
+       unsigned long arg5;
+       unsigned long arg6;
+       unsigned long ret;
+};

While this works, the argN+1 shift for 32-bit is a gotcha to learn.
Lets say arg1 was 64-bit, and my program only examined arg2. I'd need
two programs, one for 64-bit (using arg2) and 32-bit (arg3). If there
was a way not to shift arguments, I could have one program for both.
Eg, additional arg1hi, arg2hi, ... for the higher order u32s.

Brendan