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Given the following C: > > struct s { > int _1; > char _2; > }; > > typedef struct s s_t; > > union u { > int *_1; > char *_2; > }; > > __section("socket_filter/fields") int fields() { > struct t { > union { > s_t s[10]; > }; > struct { > union u u; > }; > } bar; > return bpf_core_field_exists((&bar)[1]); > } > > clang-12 generates the following instructions: > > 0000000000000000 : > ; return bpf_core_field_exists((&bar)[1]); > 0: b7 00 00 00 58 00 00 00 r0 = 88 > 1: 95 00 00 00 00 00 00 00 exit > > The weird bit is that the immediate for instruction 0 isn't 1 but 88. > Coincidentally sizeof(bar) is also 88 bytes. Thanks for the reporting. This is a compiler issue which didn't handle invalid case nicely. The following is an explanation. After macro expansion, bpf_core_field_exists((&bar)[1]); is actually __builtin_preserve_field_info((&bar)[1], BPF_FIELD_EXISTS); For BPF_FIELD_EXISTS, the first argument should be a field access. But in the above, we got an array access, (&bar)[1], internally, the compiler keeps track of the offset from the base address, and this offset is used for most other builtin kinds like FIELD_BYPE_OFFSET etc. For relative to &bar, (&bar)[1] has an offset 88. In your particular case, the code never went inside the routine to generate correct "patch_imm" (i.e., r0 = in the above) since we didn't get a field access. So we got a wrong result. I will fix this in the compiler by issuing an error so people can correct their usage. Thanks for reporting! > > $ clang-12 --version > Ubuntu clang version > 12.0.0-++20210126113614+510b3d4b3e02-1~exp1~20210126104320.178 > > I've tried clang-13 as well, same result. > > Best, > Lorenz >