Martin KaFai Lau <martin.lau@xxxxxxxxx> 于2022年12月22日周四 05:21写道： > > On 12/21/22 5:46 AM, Hao Sun wrote: > > Hi, > > > > I’ve tried something like the bellow, but soon realized that this > > won’t work because once compiler figures out `inner_map` equals > > to `val`, it can choose either reg to write into in the following > > path, meaning that this program can be rejected due to writing > > into read-only PTR_TO_BTF_ID reg, and this makes the test useless. > > hmm... I read the above a few times but I still don't quite get it. In > particular, '...can be rejected due to writing into read-only PTR_TO_BTF_ID > reg...'. Where is it writing into a read-only PTR_TO_BTF_ID reg in the > following bpf prog? Did I overlook something? > > > > > Essentially, we want two regs, one points to PTR_TO_BTD_ID, one > > points to MAP_VALUR_OR_NULL, then compare them and deref map val. > > If I read this request correctly, I guess the compiler has changed 'ret = *val' > to 'ret = *inner_map'? Thus, the verifier did not reject because it deref a > PTR_TO_BTF_ID? > Yes, and if we do "*val = 0", it's rejected due to writing to read-only PTR_TO_BTF_ID reg. > > It’s hard to implement this in C level because compilers decide > > which reg to use but not us, maybe we can just drop this test. > > Have you tried inline assembly. Something like this (untested): > > asm volatile ( > "r8 = %[val];\n" > "r9 = %[inner_map];\n" > "if r8 != r9 goto +1;\n" > "%[ret] = *(u64 *)(r8 +0);\n" > :[ret] "+r"(ret) > : [inner_map] "r"(inner_map), [val] "r"(val) > :"r8", "r9"); > This would work, didn't realize that I can inline BPF insns this way. Thanks! > Please attach the verifier output in the future. It will be easier to understand. > Will do the next time.
