Re: [RFC][PATCH 5/5] tracing/filters: Provide support for char * pointers

[Li Zefan <lizf@cn.fujitsu.com>]

Frederic Weisbecker wrote:
> Provide support for char * pointers in the filtering framework.
> Usually, char * entries are dangerous in traces because the string
> can be released whereas a pointer to it can still wait to be read from
> the ring buffer. But sometimes we can assume it's safe, like in case
> of RO data (eg: __file__ or __line__, used in bkl trace event). If
> these RO data are in a module and so is the call to the trace event,
> then it's safe, because the ring buffer will be flushed once this
> module get unloaded.
> 

The problem is we don't distinguish dangerous char * from
safe char *... They are both defined as:
	__field(char *, str)

So for those dangerous ones, a string filter still can be applied,
which will dereference those pointers.

> Now the bkl events becomes more useful. Say that you want to trace
> only the bkl use in reiserfs:
> 
> cd /debug/tracing/events/bkl/lock_kernel
> echo "file == fs/reiserfs*" > filter_regex
> cat /debug/tracing/trace
> 
> syslogd-3658  [001]  1874.661878: lock_kernel: depth: 1, fs/reiserfs/super.c:563 reiserfs_dirty_inode()
> syslogd-3658  [001]  1874.662266: lock_kernel: depth: 0, fs/reiserfs/inode.c:2695 reiserfs_write_end()
> syslogd-3658  [001]  1874.662268: lock_kernel: depth: 1, fs/reiserfs/super.c:563 reiserfs_dirty_inode()
> syslogd-3658  [001]  1874.662291: lock_kernel: depth: 0, fs/reiserfs/inode.c:2695 reiserfs_write_end()