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

[Yonghong Song <yhs@fb.com>]

On 10/18/19 9:48 AM, Andrii Nakryiko wrote:
> On 10/18/19 3:34 AM, Jiri Olsa wrote:
>> The bpftool interface stays the same, but now it's possible
>> to run it over BTF raw data, like:
> 
> Oh, great, I had similar patch laying around for a while, never got to
> cleaning it up, though, so thanks for picking this up!
> 
>>
>>     $ bpftool btf dump file /sys/kernel/btf/vmlinux
>>     libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux
> 
> We should implement this so that we don't get an extra log output with
> errors. I've been thinking about checking first few bytes of the file.
> If that matches BTF_MAGIC, then try to parse it as raw BTF, otherwise
> parse as ELF w/ BTF. Does it make sense?

Agreed, this makes sense. We should not emit errors in such cases.
One minor comment below.

> 
>>     [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 <jolsa@kernel.org>
>> ---
>>    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;
>> +	struct stat st;
>> +	FILE *f;
>> +
>> +	if (stat(file, &st))
>> +		return btf;
>> +
>> +	f = fopen(file, "rb");
>> +	if (!f)
>> +		return btf;
>> +
>> +	buf = malloc(st.st_size);
>> +	if (!buf)
>> +		goto err;
>> +
>> +	if ((size_t) st.st_size != fread(buf, 1, st.st_size, f))
>> +		goto err;
>> +
>> +	btf = btf__new(buf, st.st_size);
>> +
>> +err:

Non error case can also reach here. Let us change
label to a different name, e.g., "done"?

>> +	free(buf);
>> +	fclose(f);
>> +	return btf;
>> +}
>> +
>>    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;
>>    
>>    	if (!REQ_ARGS(2)) {
>>    		usage();
>> @@ -468,10 +500,15 @@ static int do_dump(int argc, char **argv)
>>    		btf = btf__parse_elf(*argv, NULL);
>>    		if (IS_ERR(btf)) {
>>    			err = PTR_ERR(btf);
>> -			btf = NULL;
>> -			p_err("failed to load BTF from %s: %s",
>> -			      *argv, strerror(err));
>> -			goto done;
>> +			if (err == -LIBBPF_ERRNO__FORMAT)
>> +				btf = btf__parse_raw(*argv);
>> +			if (IS_ERR(btf)) {
>> +				btf = NULL;
>> +				/* Display the original error value. */
>> +				p_err("failed to load BTF from %s: %s",
>> +				      *argv, strerror(err));
>> +				goto done;
>> +			}
>>    		}
>>    		NEXT_ARG();
>>    	} else {
>>
>