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| author | John Fastabend <john.fastabend@gmail.com> | 2020-09-24 22:58:40 +0300 |
|---|---|---|
| committer | Alexei Starovoitov <ast@kernel.org> | 2020-09-26 03:05:14 +0300 |
| commit | ba5f4cfeac77fca981b199ec7f2396a3616e5216 (patch) | |
| tree | 38b830f22adca729f67c432bef9213b81a9127c9 /include/linux/bpf.h | |
| parent | 99d4def4d08507474b250dad6345d14715a4726b (diff) | |
| download | linux-ba5f4cfeac77fca981b199ec7f2396a3616e5216.tar.xz | |
bpf: Add comment to document BTF type PTR_TO_BTF_ID_OR_NULL
The meaning of PTR_TO_BTF_ID_OR_NULL differs slightly from other types
denoted with the *_OR_NULL type. For example the types PTR_TO_SOCKET
and PTR_TO_SOCKET_OR_NULL can be used for branch analysis because the
type PTR_TO_SOCKET is guaranteed to _not_ have a null value.
In contrast PTR_TO_BTF_ID and BTF_TO_BTF_ID_OR_NULL have slightly
different meanings. A PTR_TO_BTF_TO_ID may be a pointer to NULL value,
but it is safe to read this pointer in the program context because
the program context will handle any faults. The fallout is for
PTR_TO_BTF_ID the verifier can assume reads are safe, but can not
use the type in branch analysis. Additionally, authors need to be
extra careful when passing PTR_TO_BTF_ID into helpers. In general
helpers consuming type PTR_TO_BTF_ID will need to assume it may
be null.
Seeing the above is not obvious to readers without the back knowledge
lets add a comment in the type definition.
Editorial comment, as networking and tracing programs get closer
and more tightly merged we may need to consider a new type that we
can ensure is non-null for branch analysis and also passing into
helpers.
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Lorenz Bauer <lmb@cloudflare.com>
Diffstat (limited to 'include/linux/bpf.h')
| -rw-r--r-- | include/linux/bpf.h | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/include/linux/bpf.h b/include/linux/bpf.h index d0937f1d2980..79902325bef8 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -383,8 +383,22 @@ enum bpf_reg_type { PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */ PTR_TO_TP_BUFFER, /* reg points to a writable raw tp's buffer */ PTR_TO_XDP_SOCK, /* reg points to struct xdp_sock */ - PTR_TO_BTF_ID, /* reg points to kernel struct */ - PTR_TO_BTF_ID_OR_NULL, /* reg points to kernel struct or NULL */ + /* PTR_TO_BTF_ID points to a kernel struct that does not need + * to be null checked by the BPF program. This does not imply the + * pointer is _not_ null and in practice this can easily be a null + * pointer when reading pointer chains. The assumption is program + * context will handle null pointer dereference typically via fault + * handling. The verifier must keep this in mind and can make no + * assumptions about null or non-null when doing branch analysis. + * Further, when passed into helpers the helpers can not, without + * additional context, assume the value is non-null. + */ + PTR_TO_BTF_ID, + /* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not + * been checked for null. Used primarily to inform the verifier + * an explicit null check is required for this struct. + */ + PTR_TO_BTF_ID_OR_NULL, PTR_TO_MEM, /* reg points to valid memory region */ PTR_TO_MEM_OR_NULL, /* reg points to valid memory region or NULL */ PTR_TO_RDONLY_BUF, /* reg points to a readonly buffer */ |