Subject: Re: [PATCH] bpftool: Try to read btf as raw data if elf read fails

From: Jiri Olsa <[email-masked]>
Date: Mon, 21 Oct 2019 16:02:27 +0200

On Fri, Oct 18, 2019 at 03:39:05PM -0700, Jakub Kicinski wrote:
> On Fri, 18 Oct 2019 12:34:04 +0200, Jiri Olsa wrote:
> > The bpftool interface stays the same, but now it's possible
> > to run it over BTF raw data, like:
> > 
> >   $ bpftool btf dump file /sys/kernel/btf/vmlinux
> >   libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux
> >   [1] INT '(anon)' size=4 bits_offset=0 nr_bits=32 encoding=(none)
> >   [2] INT 'long unsigned int' size=8 bits_offset=0 nr_bits=64 encoding=(none)
> >   [3] CONST '(anon)' type_id=2
> > 
> > I'm also adding err init to 0 because I was getting uninitialized
> > warnings from gcc.
> > 
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >  tools/bpf/bpftool/btf.c | 47 ++++++++++++++++++++++++++++++++++++-----
> >  1 file changed, 42 insertions(+), 5 deletions(-)
> > 
> > diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> > index 9a9376d1d3df..100fb7e02329 100644
> > --- a/tools/bpf/bpftool/btf.c
> > +++ b/tools/bpf/bpftool/btf.c
> > @@ -12,6 +12,9 @@
> >  #include <libbpf.h>
> >  #include <linux/btf.h>
> >  #include <linux/hashtable.h>
> > +#include <sys/types.h>
> > +#include <sys/stat.h>
> > +#include <unistd.h>
> >  
> >  #include "btf.h"
> >  #include "json_writer.h"
> > @@ -388,6 +391,35 @@ static int dump_btf_c(const struct btf *btf,
> >  	return err;
> >  }
> >  
> > +static struct btf *btf__parse_raw(const char *file)
> > +{
> > +	struct btf *btf = ERR_PTR(-EINVAL);
> > +	__u8 *buf = NULL;
> 
> Please drop the inits
> 
> > +	struct stat st;
> > +	FILE *f;
> > +
> > +	if (stat(file, &st))
> > +		return btf;
> 
> And return constants here
> 
> > +	f = fopen(file, "rb");
> > +	if (!f)
> > +		return btf;
> 
> and here
> 
> > +	buf = malloc(st.st_size);
> > +	if (!buf)
> > +		goto err;
> 
> and jump to the right place here.
> 
> > +	if ((size_t) st.st_size != fread(buf, 1, st.st_size, f))
> > +		goto err;
> > +
> > +	btf = btf__new(buf, st.st_size);
> > +
> > +err:
> 
> The prefix for error labels which is shared with non-error path is exit_
> 
> > +	free(buf);
> > +	fclose(f);
> > +	return btf;
> > +}
> > +

ok for all above

> >  static int do_dump(int argc, char **argv)
> >  {
> >  	struct btf *btf = NULL;
> > @@ -397,7 +429,7 @@ static int do_dump(int argc, char **argv)
> >  	__u32 btf_id = -1;
> >  	const char *src;
> >  	int fd = -1;
> > -	int err;
> > +	int err = 0;
> 
> This change looks unnecessary.

I'm getting confusing warnings from gcc about this,
but there is a code path where do_dump would return
untouched err:

  do_dump
     int err;

     } else if (is_prefix(src, "file")) {
       btf = btf__parse_elf(*argv, NULL);   // succeeds

     }

     while (argc) {
       if (is_prefix(*argv, "format")) {
       else {                                // in here
          goto done;
       }

     done:
       return err;

thanks,
jirka