On 10/7/21 5:02 PM, andrii.nakryiko@xxxxxxxxx wrote:
One interesting possibility that is now open by these changes is that it's
possible to do:
bpf_trace_printk("My fmt %s", sizeof("My fmt %s"), "blah");
and it will work as expected.
Could you explain what would be the difference vs existing bpf_printk macro?
Why these patches enable above to work ?
I haven't updated libbpf-provided helpers in
bpf_helpers.h for snprintf, seq_printf, and printk, because using
`static const char ___fmt[] = fmt;` trick is still efficient and doesn't fill
out the buffer at runtime (no copying), but it also enforces that format
string is compile-time string literal:
const char *s = NULL;
bpf_printk("hi"); /* works */
bpf_printk(s); /* compilation error */
By passing fmt directly to bpf_trace_printk() would actually compile
bpf_printk(s) above with no warnings and will not work at runtime, which is
worse user experience, IMO.
What is the example of "compile with no warning and not work at runtime" ?
