Re: [RFC dwarves] syscall functions in BTF

[Jiri Olsa <olsajiri@gmail.com>]

On Fri, Mar 10, 2023 at 12:43:31PM +0000, Alan Maguire wrote:
> On 10/03/2023 10:07, Jiri Olsa wrote:
> > hi,
> > with latest pahole fixes we get rid of some syscall functions (with
> > __x64_sys_ prefix) and it seems to fall down to 2 cases:
> > 
> > - weak syscall functions generated in kernel/sys_ni.c prevent these syscalls
> >   to be generated in BTF. The reason is the __COND_SYSCALL macro uses
> >   '__unused' for regs argument:
> > 
> >         #define __COND_SYSCALL(abi, name)                                      \
> >                __weak long __##abi##_##name(const struct pt_regs *__unused);   \
> >                __weak long __##abi##_##name(const struct pt_regs *__unused)    \
> >                {                                                               \
> >                        return sys_ni_syscall();                                \
> >                }
> > 
> >   and having weak function with different argument name will rule out the
> >   syscall from BTF functions
> > 
> >   the patch below workarounds this by using the same argument name,
> >   but I guess the real fix would be to check the whole type not just
> >   the argument name.. or ignore weak function if there's non weak one
> > 
> >   I guess there will be more cases like this in kernel
> > 
> >
> 
> Thanks for the report Jiri! I'm working on reusing the dwarves_fprintf.c
> code to use string comparisons of function prototypes (minus parameter names!)
> instead as a more robust comparison.  Hope to have something working soon..

great, I saw the patchset, will check

>  
> > - we also do not get any syscall with no arguments, because they are
> >   generated as aliases to __do_<syscall> function:
> > 
> >         $ nm ./vmlinux | grep _sys_fork
> >         ffffffff81174890 t __do_sys_fork
> >         ffffffff81174890 T __ia32_sys_fork
> >         ffffffff81174880 T __pfx___x64_sys_fork
> >         ffffffff81174890 T __x64_sys_fork
> > 
> >   with:
> >         #define __SYS_STUB0(abi, name)                                          \
> >                 long __##abi##_##name(const struct pt_regs *regs);              \
> >                 ALLOW_ERROR_INJECTION(__##abi##_##name, ERRNO);                 \
> >                 long __##abi##_##name(const struct pt_regs *regs)               \
> >                         __alias(__do_##name);
> > 
> >   the problem seems to be that there's no DWARF data for aliased symbol,
> >   so pahole won't see any __x64_sys_fork record
> >   I'm not sure how to fix this one
> > 
> 
> Is this one a new issue, or did you just spot it when looking at the other case?

I was trying to attach to all syscalls and noticed some where missing,
it looks like the alias was used in this place for few years

thanks,
jirka