bpf: Introduce bpf_can_loop #6488
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While working on bpf_arena the following monster macro had to be
used to iterate a link list:
for (struct bpf_iter_num ___it __attribute__((aligned(8),
cleanup(bpf_iter_num_destroy))),
* ___tmp = (bpf_iter_num_new(&___it, 0, (1000000)),
pos = list_entry_safe((head)->first,
typeof(*(pos)), member),
(void)bpf_iter_num_destroy,
(void *)0);
bpf_iter_num_next(&___it) && pos &&
({ ___tmp = (void *)pos->member.next; 1; });
pos = list_entry_safe((void __arena *)___tmp, typeof(*(pos)), member))
It's similar to bpf_for(), bpf_repeat() macros.
Unfortunately every "for" in normal C code needs an equivalent monster macro.
Instead, let's introduce bpf_can_loop() kfunc that acts on a hidden
bpf_iter_num, so that bpf_iter_num_new(), bpf_iter_num_destroy()
don't need to be called explicitly. It simplifies the macro to:
for (void * ___tmp = (pos = list_entry_safe((head)->first,
typeof(*(pos)), member),
(void *)0);
bpf_can_loop(0) && pos &&
({ ___tmp = (void *)pos->member.next; 1; });
pos = list_entry_safe((void __arena *)___tmp, typeof(*(pos)), member))
and can be used in any normal "for" or "while" loop, like
for (i = 0; i < cnt && bpf_can_loop(0); i++) {
The verifier recognizes that bpf_can_loop() is used in the program,
reserves additional 8 bytes of stack, zero initializes them in subprog
prologue, and passes that address to bpf_can_loop() kfunc that simply
increments the counter until it reaches BPF_MAX_LOOPS.
In the future bpf_can_loop() can be inlined to improve performance.
New instruction with the same semantics can be added,
and LLVM will finally be able to emit __builtin_memcpy, __builtin_strcmp.
bpf_can_loop() is not a substitute for bpf_for() when it's used to
iterate normal arrays or map_values.
bpf_can_loop() works well only with arena pointers that don't need
to be bounds-checked on every iteration.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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Upstream branch: e59997d |
Add a sanity test for bpf_can_loop(). Signed-off-by: Alexei Starovoitov <ast@kernel.org>
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At least one diff in series https://patchwork.kernel.org/project/netdevbpf/list/?series=830159 expired. Closing PR. |
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Pull request for series with
subject: bpf: Introduce bpf_can_loop
version: 2
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=830159