On Tue, Nov 4, 2025 at 4:40 PM Andrii Nakryiko <[email protected]> wrote: > > On Mon, Nov 3, 2025 at 3:29 AM Menglong Dong <[email protected]> wrote: > > > > On 2025/11/1 01:57, Alexei Starovoitov wrote: > > > On Thu, Oct 30, 2025 at 8:36 PM Menglong Dong <[email protected]> > > > wrote: > > > > > > > > On Fri, Oct 31, 2025 at 9:42 AM Alexei Starovoitov > > > > <[email protected]> wrote: > > > > > > > > > > On Sat, Oct 25, 2025 at 8:02 PM Menglong Dong > > > > > <[email protected]> wrote: > > > > > > > > > > > > Add BPF_TRACE_SESSION supporting to x86_64. > > > > > > invoke_bpf_session_entry and > > > > > > invoke_bpf_session_exit is introduced for this purpose. > > > > > > > > > > > > In invoke_bpf_session_entry(), we will check if the return value of > > > > > > the > > > > > > fentry is 0, and set the corresponding session flag if not. And in > > > > > > invoke_bpf_session_exit(), we will check if the corresponding flag > > > > > > is > > > > > > set. If set, the fexit will be skipped. > > > > > > > > > > > > As designed, the session flags and session cookie address is stored > > > > > > after > > > > > > the return value, and the stack look like this: > > > > > > > > > > > > cookie ptr -> 8 bytes > > > > > > session flags -> 8 bytes > > > > > > return value -> 8 bytes > > > > > > argN -> 8 bytes > > > > > > ... > > > > > > arg1 -> 8 bytes > > > > > > nr_args -> 8 bytes > > Let's look at "cookie ptr", "session flags", and "nr_args". We can > combine all of them into a single 8 byte slot: assign each session > program index 0, 1, ..., Nsession. 1 bit for entry/exit flag, few bits > for session prog index, and few more bits for nr_args, and we still > will have tons of space for some other additions in the future. From > that session program index you can calculate cookieN address to return > to user. > > And we should look whether moving nr_args into bpf_run_ctx would > actually minimize amount of trampoline assembly code, as we can > implement a bunch of stuff in pure C. (well, BPF verifier inlining is > a separate thing, but it can be mostly arch-independent, right?)
Instead of all that I have a different suggestion... how about we introduce this "session" attach type, but won't mess with trampoline and whole new session->nr_links. Instead the same prog can be added to 'fentry' list and 'fexit' list. We lose the ability to skip fexit, but I'm still not convinced it's necessary. The biggest benefit is that it will work for existing JITs and trampolines. No new complex asm will be necessary. As far as writable session_cookie ... let's add another 8 byte space to bpf_tramp_run_ctx and only allow single 'fsession' prog for a given kernel function. Again to avoid changing all trampolines. This way the feature can be implemented purely in C and no arch specific changes. It's more limited, but doesn't sound that the use case for multiple fsession-s exist. All this is on/off tracing. Not something that will be attached 24/7.
