Often we want to write tests cases that check things like bad context offset accesses. And one way to do this is to use an odd offset on, for example, a 32-bit load. This unfortunately triggers the alignment checks first on platforms that do not set CONFIG_EFFICIENT_UNALIGNED_ACCESS. So the test case see the alignment failure rather than what it was testing for. It is often not completely possible to respect the original intention of the test, or even test the same exact thing, while solving the alignment issue. Another option could have been to check the alignment after the context and other validations are performed by the verifier, but that is a non-trivial change to the verifier.
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| @@ -232,6 +232,20 @@ enum bpf_attach_type { · 231 unmodified lines | |||
| 232 | */ | 232 | */ |
| 233 | #define BPF_F_STRICT_ALIGNMENT (1U << 0) | 233 | #define BPF_F_STRICT_ALIGNMENT (1U << 0) |
| 234 | 234 | ||
| 235 | /* If BPF_F_ANY_ALIGNMENT is used in BPF_PROF_LOAD command, the | ||
| 236 | * verifier will allow any alignment whatsoever. On platforms | ||
| 237 | * with strict alignment requirements for loads ands stores (such | ||
| 238 | * as sparc and mips) the verifier validates that all loads and | ||
| 239 | * stores provably follow this requirement. This flag turns that | ||
| 240 | * checking and enforcement off. | ||
| 241 | * | ||
| 242 | * It is mostly used for testing when we want to validate the | ||
| 243 | * context and memory access aspects of the verifier, but because | ||
| 244 | * of an unaligned access the alignment check would trigger before | ||
| 245 | * the one we are interested in. | ||
| 246 | */ | ||
| 247 | #define BPF_F_ANY_ALIGNMENT (1U << 1) | ||
| 248 | |||
| 235 | /* when bpf_ldimm64->src_reg == BPF_PSEUDO_MAP_FD, bpf_ldimm64->imm == fd */ | 249 | /* when bpf_ldimm64->src_reg == BPF_PSEUDO_MAP_FD, bpf_ldimm64->imm == fd */ |
| 236 | #define BPF_PSEUDO_MAP_FD 1 | 250 | #define BPF_PSEUDO_MAP_FD 1 |
| 237 | 251 | ||