On Fri, 25 Oct 2019 09:53:07 -0700, Andrii Nakryiko wrote:
> On Fri, Oct 25, 2019 at 9:31 AM Jakub Kicinski wrote:
> > On Fri, 25 Oct 2019 05:01:17 +0000, Andrii Nakryiko wrote:
> > > >> +static bool is_btf_raw(const char *file)
> > > >> +{
> > > >> + __u16 magic = 0;
> > > >> + int fd;
> > > >> +
> > > >> + fd = open(file, O_RDONLY);
> > > >> + if (fd < 0)
> > > >> + return false;
> > > >> +
> > > >> + read(fd, &magic, sizeof(magic));
> > > >> + close(fd);
> > > >> + return magic == BTF_MAGIC;
> > > >
> > > > Isn't it suspicious to read() 2 bytes into an u16 and compare to a
> > > > constant like endianness doesn't matter? Quick grep doesn't reveal
> > > > BTF_MAGIC being endian-aware..
> > >
> > > Right now we support only loading BTF in native endianness, so I think
> > > this should do. If we ever add ability to load non-native endianness,
> > > then we'll have to adjust this.
> >
> > This doesn't do native endianness, this does LE-only. It will not work
> > on BE machines.
>
> How is this LE-only? You have 2 first bytes in BE-encoding on BE
> machines and in LE-encoding on LE machines. You read those two bytes
> as is into u16, then do comparison to u16. Given all of that is
> supposed to be in native encoding, this will work. What am I missing?
I see so the on-disk format depends on the endianness of the writer?
You write it broken and therefore you can also read it broken.
And cross compilation etc. is, I presume, on the TODO list?
Got it.
