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vzdBdAZfPUWquE3mt-z7BD9YAKscrwsp Andrii Nakryiko writes: > On Sat, Apr 27, 2024 at 3:51=E2=80=AFPM Cupertino Miranda > wrote: >> >> >> Alexei Starovoitov writes: >> >> > On Fri, Apr 26, 2024 at 9:12=E2=80=AFAM Andrii Nakryiko >> > wrote: >> >> >> >> On Fri, Apr 26, 2024 at 3:20=E2=80=AFAM Cupertino Miranda >> >> wrote: >> >> > >> >> > >> >> > Andrii Nakryiko writes: >> >> > >> >> > > On Wed, Apr 24, 2024 at 3:41=E2=80=AFPM Cupertino Miranda >> >> > > wrote: >> >> > >> >> >> > >> Split range computation checks in its own function, isolating pe= ssimitic >> >> > >> range set for dst_reg and failing return to a single point. >> >> > >> >> >> > >> Signed-off-by: Cupertino Miranda >> >> > >> Cc: Yonghong Song >> >> > >> Cc: Alexei Starovoitov >> >> > >> Cc: David Faust >> >> > >> Cc: Jose Marchesi >> >> > >> Cc: Elena Zannoni >> >> > >> --- >> >> > >> kernel/bpf/verifier.c | 141 +++++++++++++++++++++++------------= ------- >> >> > >> 1 file changed, 77 insertions(+), 64 deletions(-) >> >> > >> >> >> > > >> >> > > I know you are moving around pre-existing code, so a bunch of nit= s >> >> > > below are to pre-existing code, but let's use this as an opportun= ity >> >> > > to clean it up a bit. >> >> > > >> >> > >> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c >> >> > >> index 6fe641c8ae33..829a12d263a5 100644 >> >> > >> --- a/kernel/bpf/verifier.c >> >> > >> +++ b/kernel/bpf/verifier.c >> >> > >> @@ -13695,6 +13695,82 @@ static void scalar_min_max_arsh(struct = bpf_reg_state *dst_reg, >> >> > >> __update_reg_bounds(dst_reg); >> >> > >> } >> >> > >> >> >> > >> +static bool is_const_reg_and_valid(struct bpf_reg_state reg, bo= ol alu32, >> >> > > >> >> > > hm.. why passing reg_state by value? Use pointer? >> >> > > >> >> > Someone mentioned this in a review already and I forgot to change i= t. >> >> > Apologies if I did not reply on this. >> >> > >> >> > The reason why I pass by value, is more of an approach to programmi= ng. >> >> > I do it as guarantee to the caller that there is no mutation of >> >> > the value. >> >> > If it is better or worst from a performance point of view it is >> >> > arguable, since although it might appear to copy the value it also = provides >> >> > more information to the compiler of the intent of the callee functi= on, >> >> > allowing it to optimize further. >> >> > I personally would leave the copy by value, but I understand if you= want >> >> > to keep having the same code style. >> >> >> >> It's a pretty big 120-byte structure, so maybe the compiler can >> >> optimize it very well, but I'd still be concerned. Hopefully it can >> >> optimize well even with (const) pointer, if inlining. >> >> >> >> But I do insist, if you look at (most? I haven't checked every single >> >> function, of course) other uses in verifier.c, we pass things like >> >> that by pointer. I understand the desire to specify the intent to not >> >> modify it, but that's why you are passing `const struct bpf_reg_state >> >> *reg`, so I think you don't lose anything with that. >> Well, the const will only guard the pointer from mutating, not the data >> pointed by it. > > I didn't propose marking pointer const, but mark pointee type as const: > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 4e474ef44e9c..de2bc6fa15da 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -363,12 +363,14 @@ __printf(2, 3) static void verbose(void > *private_data, const char *fmt, ...) > } > > static void verbose_invalid_scalar(struct bpf_verifier_env *env, > - struct bpf_reg_state *reg, > + const struct bpf_reg_state *reg, > struct bpf_retval_range range, > const char *ctx, > const char *reg_name) > { > bool unknown =3D true; > > + reg->smin_value =3D 0x1234; > + > verbose(env, "%s the register %s has", ctx, reg_name); > if (reg->smin_value > S64_MIN) { > verbose(env, " smin=3D%lld", reg->smin_value); > > $ make > > ... > > /data/users/andriin/linux/kernel/bpf/verifier.c: In function > =E2=80=98verbose_invalid_scalar=E2=80=99: > /data/users/andriin/linux/kernel/bpf/verifier.c:372:25: error: > assignment of member =E2=80=98smin_value=E2=80=99 in read-only object > 372 | reg->smin_value =3D 0x1234; > | ^ > > ... > > Works as it logically should. > Your right, pointer is better. I should have validated that myself. Apologies for the noise. Please disregard all I said. >> >> > >> > +1 >> > that "struct bpf_reg_state src_reg" code was written 7 years ago >> > when bpf_reg_state was small. >> > We definitely need to fix it. It might even bring >> > a noticeable runtime improvement. >> >> I forgot to reply to Andrii. >> >> I will change the function prototype to pass the pointer instead. >> In any case, please allow me to express my concerns once again, and >> explain why I do it. >> >> As a general practice, I personally will only copy a pointer to a >> function if there is the intent to allow the function to change the >> content of the pointed data. > > I'm not sure why you have this preconception that passing something by > pointer is only for mutation. C language has a straightforward way to > express "this is not going to be changed" with const. You can > circumvent this, of course, but that's an entirely different story. > >> >> In my understanding, it is easier for the compiler to optimize both the >> caller and the callee when there are less side-effects from that >> function call such as a possible memory clobbering. >> >> Since these particular functions are leaf functions (not calling anywher= e), >> it should be relatively easy for the compiler to infer that the actual >> copy is not needed and will likely just inline those calls, resulting in >> lots of code being eliminated, which will remove any apparent copies. >> >> I checked the asm file for verifier.c and everything below >> adjust_scalar_min_max_vals including itself is inlined, making it >> totally irrelevant if you copy the data or the pointer, since the >> compiler will identify that the content refers to the same data and all >> copies will be classified and removed as dead-code. >> >> All the pointer passing in any context in verifier.c, to my eyes, is mor= e >> of a software defect then a virtue. >> When there is an actual proven benefit, I am all for it, but not in all >> cases. >> >> I had to express my concerns on this and will never speak of it again. >> :) >> >> Thanks you all for the reviews. I will prepare a new version on Monday.