RE: [PATCH bpf-next v2 2/2] bpf: Replace callers of BPF_CAST_CALL with proper function typedef
From: David Laight <hidden>
Date: 2021-10-02 16:09:25
Also in:
bpf, linux-hardening, lkml
From: Kees Cook
Sent: 29 September 2021 00:10
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In order to keep ahead of cases in the kernel where Control Flow Integrity (CFI) may trip over function call casts, enabling -Wcast-function-type is helpful. To that end, BPF_CAST_CALL causes various warnings and is one of the last places in the kernel triggering this warning.
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quoted hunk ↗ jump to hunk
-static int bpf_for_each_array_elem(struct bpf_map *map, void *callback_fn, +static int bpf_for_each_array_elem(struct bpf_map *map, bpf_callback_t callback_fn, void *callback_ctx, u64 flags) { u32 i, key, num_elems = 0;@@ -668,9 +668,8 @@ static int bpf_for_each_array_elem(struct bpf_map *map, void *callback_fn, val = array->value + array->elem_size * i; num_elems++; key = i; - ret = BPF_CAST_CALL(callback_fn)((u64)(long)map, - (u64)(long)&key, (u64)(long)val, - (u64)(long)callback_ctx, 0); + ret = callback_fn((u64)(long)map, (u64)(long)&key, + (u64)(long)val, (u64)(long)callback_ctx, 0); /* return value: 0 - continue, 1 - stop and return */ if (ret) break;
This is still entirely horrid and potentially error prone. While a callback function seems a nice idea the code is almost always better and much easier to read if some kind of iterator function is used so that the calling code is just a simple loop. This is true even if you need a #define for the loop end. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)