Question: perf dso support for /proc/kallsyms

[leo . yan]

Hi all,

Now I found that if use the command 'perf script' for Arm CoreSight trace
data, it fails to parse kernel symbols if we don't specify kernel vmlinux
file.   So when we don't specify kernel symbol files then perf tool will
roll back to use /proc/kallsyms for kernel symbols parsing, as result it will
run into below flow:

  thread__find_addr_map(thread, cpumode, MAP__FUNCTION, address, &al);
  map__load(al.map);
  dso__data_read_offset(al.map->dso, machine, offset, buffer, size);
    `-> data_read_offset()

I can observe the function data_read_offset() returns failure, this is caused
by checking the offset sanity "if (offset > dso->data.file_size)"  (I pasted
the whole function code at below in case you want to get more context for it),
but if perf use "/proc/kallsyms" to load kernel symbols, the variable
'dso->data.file_size' will be set to zero thus the sanity checking always
thinks the offset is out of the file size bound.

Now I still don't understand how the dso/map support "/proc/kallsyms" and
have no idea to fix this issue, though I spent some time to look into it.

Could you give some suggestion for this?  Or even better if you have fixing
for this, I am glad to test at my side.

static ssize_t data_read_offset(struct dso *dso, struct machine *machine,
                                u64 offset, u8 *data, ssize_t size)
{
        if (data_file_size(dso, machine))
                return -1;

        /* Check the offset sanity. */
        if (offset > dso->data.file_size)
                return -1;

        if (offset + size < offset)
                return -1;

        return cached_read(dso, machine, offset, data, size);
}

Thanks,
Leo Yan