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By default, the subprog generated by the verifier to handle a thrown exception hardcodes a return value of 0. To allow user-defined logic and modification of the return value when an exception is thrown, introduce the 'exception_callback:' declaration tag, which marks a callback as the default exception handler for the program. The format of the declaration tag is 'exception_callback:<value>', where <value> is the name of the exception callback. Each main program can be tagged using this BTF declaratiion tag to associate it with an exception callback. In case the tag is absent, the default callback is used. As such, the exception callback cannot be modified at runtime, only set during verification. Allowing modification of the callback for the current program execution at runtime leads to issues when the programs begin to nest, as any per-CPU state maintaing this information will have to be saved and restored. We don't want it to stay in bpf_prog_aux as this takes a global effect for all programs. An alternative solution is spilling the callback pointer at a known location on the program stack on entry, and then passing this location to bpf_throw as a parameter. However, since exceptions are geared more towards a use case where they are ideally never invoked, optimizing for this use case and adding to the complexity has diminishing returns. Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> Link: https://lore.kernel.org/r/20230912233214.1518551-7-memxor@gmail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org>
211 lines
7.5 KiB
C
211 lines
7.5 KiB
C
#ifndef __BPF_EXPERIMENTAL__
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#define __BPF_EXPERIMENTAL__
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#include <vmlinux.h>
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#include <bpf/bpf_tracing.h>
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#include <bpf/bpf_helpers.h>
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#include <bpf/bpf_core_read.h>
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#define __contains(name, node) __attribute__((btf_decl_tag("contains:" #name ":" #node)))
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/* Description
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* Allocates an object of the type represented by 'local_type_id' in
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* program BTF. User may use the bpf_core_type_id_local macro to pass the
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* type ID of a struct in program BTF.
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*
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* The 'local_type_id' parameter must be a known constant.
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* The 'meta' parameter is rewritten by the verifier, no need for BPF
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* program to set it.
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* Returns
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* A pointer to an object of the type corresponding to the passed in
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* 'local_type_id', or NULL on failure.
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*/
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extern void *bpf_obj_new_impl(__u64 local_type_id, void *meta) __ksym;
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/* Convenience macro to wrap over bpf_obj_new_impl */
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#define bpf_obj_new(type) ((type *)bpf_obj_new_impl(bpf_core_type_id_local(type), NULL))
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/* Description
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* Free an allocated object. All fields of the object that require
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* destruction will be destructed before the storage is freed.
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*
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* The 'meta' parameter is rewritten by the verifier, no need for BPF
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* program to set it.
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* Returns
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* Void.
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*/
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extern void bpf_obj_drop_impl(void *kptr, void *meta) __ksym;
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/* Convenience macro to wrap over bpf_obj_drop_impl */
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#define bpf_obj_drop(kptr) bpf_obj_drop_impl(kptr, NULL)
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/* Description
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* Increment the refcount on a refcounted local kptr, turning the
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* non-owning reference input into an owning reference in the process.
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*
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* The 'meta' parameter is rewritten by the verifier, no need for BPF
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* program to set it.
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* Returns
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* An owning reference to the object pointed to by 'kptr'
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*/
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extern void *bpf_refcount_acquire_impl(void *kptr, void *meta) __ksym;
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/* Convenience macro to wrap over bpf_refcount_acquire_impl */
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#define bpf_refcount_acquire(kptr) bpf_refcount_acquire_impl(kptr, NULL)
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/* Description
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* Add a new entry to the beginning of the BPF linked list.
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*
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* The 'meta' and 'off' parameters are rewritten by the verifier, no need
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* for BPF programs to set them
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* Returns
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* 0 if the node was successfully added
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* -EINVAL if the node wasn't added because it's already in a list
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*/
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extern int bpf_list_push_front_impl(struct bpf_list_head *head,
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struct bpf_list_node *node,
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void *meta, __u64 off) __ksym;
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/* Convenience macro to wrap over bpf_list_push_front_impl */
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#define bpf_list_push_front(head, node) bpf_list_push_front_impl(head, node, NULL, 0)
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/* Description
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* Add a new entry to the end of the BPF linked list.
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*
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* The 'meta' and 'off' parameters are rewritten by the verifier, no need
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* for BPF programs to set them
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* Returns
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* 0 if the node was successfully added
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* -EINVAL if the node wasn't added because it's already in a list
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*/
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extern int bpf_list_push_back_impl(struct bpf_list_head *head,
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struct bpf_list_node *node,
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void *meta, __u64 off) __ksym;
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/* Convenience macro to wrap over bpf_list_push_back_impl */
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#define bpf_list_push_back(head, node) bpf_list_push_back_impl(head, node, NULL, 0)
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/* Description
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* Remove the entry at the beginning of the BPF linked list.
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* Returns
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* Pointer to bpf_list_node of deleted entry, or NULL if list is empty.
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*/
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extern struct bpf_list_node *bpf_list_pop_front(struct bpf_list_head *head) __ksym;
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/* Description
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* Remove the entry at the end of the BPF linked list.
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* Returns
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* Pointer to bpf_list_node of deleted entry, or NULL if list is empty.
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*/
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extern struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head) __ksym;
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/* Description
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* Remove 'node' from rbtree with root 'root'
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* Returns
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* Pointer to the removed node, or NULL if 'root' didn't contain 'node'
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*/
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extern struct bpf_rb_node *bpf_rbtree_remove(struct bpf_rb_root *root,
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struct bpf_rb_node *node) __ksym;
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/* Description
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* Add 'node' to rbtree with root 'root' using comparator 'less'
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*
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* The 'meta' and 'off' parameters are rewritten by the verifier, no need
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* for BPF programs to set them
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* Returns
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* 0 if the node was successfully added
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* -EINVAL if the node wasn't added because it's already in a tree
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*/
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extern int bpf_rbtree_add_impl(struct bpf_rb_root *root, struct bpf_rb_node *node,
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bool (less)(struct bpf_rb_node *a, const struct bpf_rb_node *b),
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void *meta, __u64 off) __ksym;
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/* Convenience macro to wrap over bpf_rbtree_add_impl */
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#define bpf_rbtree_add(head, node, less) bpf_rbtree_add_impl(head, node, less, NULL, 0)
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/* Description
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* Return the first (leftmost) node in input tree
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* Returns
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* Pointer to the node, which is _not_ removed from the tree. If the tree
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* contains no nodes, returns NULL.
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*/
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extern struct bpf_rb_node *bpf_rbtree_first(struct bpf_rb_root *root) __ksym;
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/* Description
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* Allocates a percpu object of the type represented by 'local_type_id' in
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* program BTF. User may use the bpf_core_type_id_local macro to pass the
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* type ID of a struct in program BTF.
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*
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* The 'local_type_id' parameter must be a known constant.
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* The 'meta' parameter is rewritten by the verifier, no need for BPF
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* program to set it.
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* Returns
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* A pointer to a percpu object of the type corresponding to the passed in
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* 'local_type_id', or NULL on failure.
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*/
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extern void *bpf_percpu_obj_new_impl(__u64 local_type_id, void *meta) __ksym;
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/* Convenience macro to wrap over bpf_percpu_obj_new_impl */
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#define bpf_percpu_obj_new(type) ((type __percpu_kptr *)bpf_percpu_obj_new_impl(bpf_core_type_id_local(type), NULL))
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/* Description
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* Free an allocated percpu object. All fields of the object that require
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* destruction will be destructed before the storage is freed.
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*
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* The 'meta' parameter is rewritten by the verifier, no need for BPF
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* program to set it.
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* Returns
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* Void.
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*/
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extern void bpf_percpu_obj_drop_impl(void *kptr, void *meta) __ksym;
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/* Convenience macro to wrap over bpf_obj_drop_impl */
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#define bpf_percpu_obj_drop(kptr) bpf_percpu_obj_drop_impl(kptr, NULL)
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/* Description
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* Throw a BPF exception from the program, immediately terminating its
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* execution and unwinding the stack. The supplied 'cookie' parameter
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* will be the return value of the program when an exception is thrown,
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* and the default exception callback is used. Otherwise, if an exception
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* callback is set using the '__exception_cb(callback)' declaration tag
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* on the main program, the 'cookie' parameter will be the callback's only
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* input argument.
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*
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* Thus, in case of default exception callback, 'cookie' is subjected to
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* constraints on the program's return value (as with R0 on exit).
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* Otherwise, the return value of the marked exception callback will be
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* subjected to the same checks.
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*
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* Note that throwing an exception with lingering resources (locks,
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* references, etc.) will lead to a verification error.
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*
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* Note that callbacks *cannot* call this helper.
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* Returns
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* Never.
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* Throws
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* An exception with the specified 'cookie' value.
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*/
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extern void bpf_throw(u64 cookie) __ksym;
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/* This macro must be used to mark the exception callback corresponding to the
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* main program. For example:
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*
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* int exception_cb(u64 cookie) {
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* return cookie;
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* }
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*
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* SEC("tc")
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* __exception_cb(exception_cb)
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* int main_prog(struct __sk_buff *ctx) {
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* ...
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* return TC_ACT_OK;
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* }
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*
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* Here, exception callback for the main program will be 'exception_cb'. Note
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* that this attribute can only be used once, and multiple exception callbacks
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* specified for the main program will lead to verification error.
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*/
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#define __exception_cb(name) __attribute__((btf_decl_tag("exception_callback:" #name)))
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#endif
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