On Wed, Aug 25, 2021 at 12:58 PM Dave Marchevsky <davemarchevsky@xxxxxx> wrote: > > Instead of being a thin wrapper which calls into bpf_trace_printk, > libbpf's bpf_printk convenience macro now chooses between > bpf_trace_printk and bpf_trace_vprintk. If the arg count (excluding > format string) is >3, use bpf_trace_vprintk, otherwise use the older > helper. > > The motivation behind this added complexity - instead of migrating > entirely to bpf_trace_vprintk - is to maintain good developer experience > for users compiling against new libbpf but running on older kernels. > Users who are passing <=3 args to bpf_printk will see no change in their > bytecode. > > __bpf_vprintk functions similarly to BPF_SEQ_PRINTF and BPF_SNPRINTF > macros elsewhere in the file - it allows use of bpf_trace_vprintk > without manual conversion of varargs to u64 array. Previous > implementation of bpf_printk macro is moved to __bpf_printk for use by > the new implementation. > > This does change behavior of bpf_printk calls with >3 args in the "new > libbpf, old kernels" scenario. On my system, using a clang built from > recent upstream sources (14.0.0 https://github.com/llvm/llvm-project.git > 50b62731452cb83979bbf3c06e828d26a4698dca), attempting to use 4 args to > __bpf_printk (old impl) results in a compile-time error: > > progs/trace_printk.c:21:21: error: too many args to 0x6cdf4b8: i64 = Constant<6> > trace_printk_ret = __bpf_printk("testing,testing %d %d %d %d\n", and with a new bpf_printk it will compile to use bpf_trace_vprintk and gets rejected during load on old kernels, right? That will be the case for any clang. It's fine. Would be good to clarify the commit log. > I was able to replicate this behavior with an older clang as well. When > the format string has >3 format specifiers, there is no output to the > trace_pipe in either case. I don't understand this paragraph. What are the cases? > After this patch, using bpf_printk with 4 args would result in a > trace_vprintk helper call being emitted and a load-time failure on older > kernels. right. > +#define __bpf_printk(fmt, ...) \ > +({ \ > + char ____fmt[] = fmt; \ Andrii was suggesting to make it const while we're at it, but that could be done in a follow up.
