Re: perf probe: request: Better message when no debug info is found on vmlinux

[Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>]

(2014/08/15 0:33), Arnaldo Carvalho de Melo wrote:
> Hi Masami,
> 
> 	I was trying to figure out why sys_perf_event_open was returning
> EACCES when trying to record, as a non priviledged user, a 'ping'
> process started by this user, while it has no problem doing the same
> thing if the monitored process is 'sleep' (reusing of ptrace perms seems
> to be what is causing this, but I digress), when I stumbled in something
> annoying:
> 
>   [root@ssdandy ~]# perf probe -v -L icmp_rcv
>   Looking at the vmlinux_path (6 entries long)
>   Using /lib/modules/3.16.0+/build/vmlinux for symbols
>   Failed to open debuginfo file.
>     Error: Failed to show lines. (-2)
>   [root@ssdandy ~]# 
> 
> Huh? Why? So I went to look to see why it was "failing to open debuginfo
> file":
> 
>   [root@ssdandy ~]# ls -la /lib/modules/3.16.0+/build/vmlinux
>   -rwxrwxr-x. 3 acme acme 22505817 Aug 13 11:54
>   /lib/modules/3.16.0+/build/vmlinux
>   [root@ssdandy ~]# ls -la /lib/modules/3.16.0+/build
>   lrwxrwxrwx. 1 root root 29 Aug 13 12:10 /lib/modules/3.16.0+/build ->
>   /home/acme/git/build/v3.16.0+
> 
> The file is there, but... I forgot I had disabled CONFIG_DEBUG_INFO on
> this machine:
> 
>   [root@ssdandy ~]# ls -la /lib/modules/3.16.0+/build/.config
>   -rw-rw-r--. 1 acme acme 136217 Aug 13 11:40
>   /lib/modules/3.16.0+/build/.config
>   [root@ssdandy ~]# grep CONFIG_DEBUG_INFO
>   /lib/modules/3.16.0+/build/.config
>   # CONFIG_DEBUG_INFO is not set
>   [root@ssdandy ~]# 
> 
> So, only the CFI ELF section was there, not the other with the -g stuff:
> 
>   [root@ssdandy ~]# readelf -SW /lib/modules/3.16.0+/build/vmlinux | grep \.debug_
>     [29] .debug_frame  PROGBITS  0000000000000000 1266030 0021d0 00  0   0  8
>   [root@ssdandy ~]#
> 
> So, I suggest that we improve that error message to be more clear:
> 
>   [root@ssdandy ~]# perf probe -L icmp_rcv
>   Failed to open debuginfo file.
>     Error: The /lib/modules/3.16.0+/build/vmlinux file has no
>            debugging information, rebuild with CONFIG_DEBUG_INFO=y
> 
>   [root@ssdandy ~]# 
> 
> Since it knows it is a kernel image file, and that the config option to
> enable it is CONFIG_DEBUG_INFO.

OK, it seems better for me too.
It will be a bit different message, but I'm sure to mention
that config option for users.

Thank you!

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com