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bpf: Add comment to document BTF type PTR_TO_BTF_ID_OR_NULL

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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>
This commit is contained in:
John Fastabend 2020-09-24 12:58:40 -07:00 committed by Alexei Starovoitov
parent 99d4def4d0
commit ba5f4cfeac
1 changed files with 16 additions and 2 deletions
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18
include/linux/bpf.h
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@ -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 */