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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>
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@ -383,8 +383,22 @@ enum bpf_reg_type {
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PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */
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PTR_TO_TP_BUFFER, /* reg points to a writable raw tp's buffer */
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PTR_TO_XDP_SOCK, /* reg points to struct xdp_sock */
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PTR_TO_BTF_ID, /* reg points to kernel struct */
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PTR_TO_BTF_ID_OR_NULL, /* reg points to kernel struct or NULL */
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/* PTR_TO_BTF_ID points to a kernel struct that does not need
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* to be null checked by the BPF program. This does not imply the
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* pointer is _not_ null and in practice this can easily be a null
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* pointer when reading pointer chains. The assumption is program
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* context will handle null pointer dereference typically via fault
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* handling. The verifier must keep this in mind and can make no
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* assumptions about null or non-null when doing branch analysis.
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* Further, when passed into helpers the helpers can not, without
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* additional context, assume the value is non-null.
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*/
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PTR_TO_BTF_ID,
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/* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not
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* been checked for null. Used primarily to inform the verifier
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* an explicit null check is required for this struct.
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*/
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PTR_TO_BTF_ID_OR_NULL,
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PTR_TO_MEM, /* reg points to valid memory region */
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PTR_TO_MEM_OR_NULL, /* reg points to valid memory region or NULL */
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PTR_TO_RDONLY_BUF, /* reg points to a readonly buffer */
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