sys/linux: describe call to BPF helper bpf_ringbuf_reserve

Describe a full call to bpf_ringbuf_reserve, using the map type created
in the previous commit. The test corresponds to this simple line:

     u64 *e;
     e = bpf_ringbuf_reserve(&rb, sizeof(*e), 0);
     [...]
     return e;

The pointer returned by bpf_ringbuf_reserve is kept in R9. The goal is
to keep it around so we can use it in other instructions later; several
other helpers take a pointer to ringbuf data as argument. There's of
course a risk that some instruction in between will clobber R9, but I
don't know another way. R9 is at least safe for calls (only R1--R5 get
clobbered).

We expect the program loading to fail with EINVAL because we never
release the reference to the ringbuf data. The verifier will therefore
reject the program with:

    0: R1=ctx(off=0,imm=0) R10=fp0
    0: (18) r0 = 0x0                      ; R0_w=0
    2: (18) r1 = 0xffff984e4b55da00       ; R1_w=map_ptr(off=0,ks=0,vs=0,imm=0)
    4: (b7) r2 = 20                       ; R2_w=20
    5: (b7) r3 = 0                        ; R3_w=0
    6: (85) call bpf_ringbuf_reserve#131  ; R0_w=ringbuf_mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) refs=2
    7: (bf) r9 = r0                       ; R0_w=ringbuf_mem_or_null(id=2,ref_obj_id=2,off=0,imm=0)
                                            R9_w=ringbuf_mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) refs=2
    8: (95) exit
    Unreleased reference id=2 alloc_insn=6

Signed-off-by: Paul Chaignon <paul.chaignon@gmail.com>

0 files changed, 0 insertions, 0 deletions