• Daniel Borkmann's avatar
    bpf: simplify narrower ctx access · f96da094
    Daniel Borkmann authored
    This work tries to make the semantics and code around the
    narrower ctx access a bit easier to follow. Right now
    everything is done inside the .is_valid_access(). Offset
    matching is done differently for read/write types, meaning
    writes don't support narrower access and thus matching only
    on offsetof(struct foo, bar) is enough whereas for read
    case that supports narrower access we must check for
    offsetof(struct foo, bar) + offsetof(struct foo, bar) +
    sizeof(<bar>) - 1 for each of the cases. For read cases of
    individual members that don't support narrower access (like
    packet pointers or skb->cb[] case which has its own narrow
    access logic), we check as usual only offsetof(struct foo,
    bar) like in write case. Then, for the case where narrower
    access is allowed, we also need to set the aux info for the
    access. Meaning, ctx_field_size and converted_op_size have
    to be set. First is the original field size e.g. sizeof(<bar>)
    as in above example from the user facing ctx, and latter
    one is the target size after actual rewrite happened, thus
    for the kernel facing ctx. Also here we need the range match
    and we need to keep track changing convert_ctx_access() and
    converted_op_size from is_valid_access() as both are not at
    the same location.
    
    We can simplify the code a bit: check_ctx_access() becomes
    simpler in that we only store ctx_field_size as a meta data
    and later in convert_ctx_accesses() we fetch the target_size
    right from the location where we do convert. Should the verifier
    be misconfigured we do reject for BPF_WRITE cases or target_size
    that are not provided. For the subsystems, we always work on
    ranges in is_valid_access() and add small helpers for ranges
    and narrow access, convert_ctx_accesses() sets target_size
    for the relevant instruction.
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
    Cc: Yonghong Song <yhs@fb.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    f96da094
bpf_trace.c 16.6 KB