Messages in this thread | | | Date | Wed, 28 Jan 2015 17:44:40 -0300 | From | Arnaldo Carvalho de Melo <> | Subject | Re: [PATCH v2 linux-trace 4/8] samples: bpf: simple tracing example in C |
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Em Wed, Jan 28, 2015 at 08:42:29AM -0800, Alexei Starovoitov escreveu: > On Wed, Jan 28, 2015 at 8:25 AM, Arnaldo Carvalho de Melo > <acme@kernel.org> wrote: > > Em Wed, Jan 28, 2015 at 01:24:15PM -0300, Arnaldo Carvalho de Melo escreveu: > >> Em Tue, Jan 27, 2015 at 08:06:09PM -0800, Alexei Starovoitov escreveu: > >> > + if (bpf_memcmp(dev->name, devname, 2) == 0)
> >> I'm only starting to look at all this, so bear with me... But why do we > >> need to have it as "bpf_memcmp"? Can't we simply use it as "memcmp" and > >> have it use the right function?
> >> Less typing, perhaps we would need to have a:
> >> #define memcmp bpf_memcmp(s1, s2, n) bpf_memcmp(s1, s2, n)
> > Argh, like this:
> > #define memcmp(s1, s2, n) bpf_memcmp(s1, s2, n)
> >> in bpf_helpers.h to have it work?
> yes, that will work just fine. > Since it's an example I made it explicit that bpf_memcmp() > has memcmp() semantics, but little bit different: > int bpf_memcmp(void *unsafe_ptr, void *safe_ptr, int size)
Not knowing about the safe/unsafe pointers (at this point in my conceptual eBPF learning process), I would think that it would be easier to understand if it would reuse another well known idiom:
#define memcmp_from_user(kernel, user, n) bpf_memcmp(user, kernel, n)
That would be similar to:
copy_from_user(void *to, const void __user *from, unsigned long n)
But here, again bear with me, I'm just brainstorming, as from just looking at:
bpf_memcmp(a, b, n)
I don't reuse anything I've learned before trying to understand eBPF, not I see any well known marker (__user) that would help me understand that that pointer needs special treatment/belongs to a different "domain".
> meaning that one of the pointers can point anywhere and > the function will be doing probe_kernel_read() underneath > similar to bpf_fetch_*() helpers.
> If it was plain memcmp() it would give a wrong impression > that vanilla memcmp() can be used.
Since that is not the case, I agree that the 'memcmp' semantic can't be used, as the two pointers are not on the same "domain", so to say.
> In general the programs cannot use any library functions > outside of helpers defined in uapi/linux/bpf.h > > bpf_fetch_*() helpers are also explicit in examples. > If one need to do a lot of pointer walking, then macro like > #define D(P) ((typeof(P))bpf_fetch_ptr(&P)) > would be easier to use: p = D(D(skb->dev)->ifalias) > multiple pointer derefs would look more natural...
And if possible, i.e. if the eBPF compiler would take care of that somehow, would indeed be preferred as it looks more natural :-)
- Arnaldo
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