[DTrace-devel] [PATCH] rawfbt: new provider

[Alan Maguire]

On 06/12/2024 16:33, Kris Van Hees wrote:
> On Fri, Dec 06, 2024 at 10:35:50AM +0000, Alan Maguire wrote:
>> On 05/12/2024 19:42, Kris Van Hees wrote:
>>> This provider provides access to all kprobe-based probes that are
>>> available on the system.  This includes any compiler-generated
>>> optimized variants of functions, named <func>.<suffix>.
>>>
>>
>> This is great! Having a rawfbt allows users to still trace cases
>> where fprobe can't currently handle.
>>
>> On the .suffix; I get that this is intended to be a raw provider, but
>> would it be better for stability to expose these functions as "func"
>> rather than "func.suffix"? It becomes difficult to write portable
>> scripts when suffixes are included, because wildcarding
>>
>> rawfbt::func*:entry
>>
>> ...to catch both suffixed and non-suffixed variants may end up tracing
>> func, func.suffix but also func2 etc.
> 
> The trouble is that exposing <func>.<suffix> as <func> will mean that you
> automatically end up trying to probe *any* <func>.<suffix> whenever you
> place a probe on <func> using rawfbt, which is not necessarily what you may
> want.  And there would not be any way to avoid that.
> 
> But by not collapsing them, you have the ability to catch them all anyway,
> if you want, by specifying <func>, <func>.* instead of <func>.
>

I tried that with the rawfbt code but got:

# dtrace -n 'rawfbt::xfs_cleanup_inode.*:entry {}'
dtrace: description 'rawfbt::xfs_cleanup_inode.*:entry ' matched 1 probe
^C

This matches xfs_cleanup_inode.isra.0, looks good so far. However:

# dtrace -n
'rawfbt::xfs_cleanup_inode:entry,rawfbt::xfs_cleanup_inode.*:entry {}'
dtrace: invalid probe specifier
rawfbt::xfs_cfs_cleanup_inode:entry,rawfbt::xfs_cleanup_inode.*:entry
{}: probe description rawfbt::xfs_cfs_cleanup_inode:entry does not match
any probes

So the absence of the non-suffixed function (which we included for
portability) triggers the failure. So looks like it will be necessary to
add -Z:

# /sbin/dtrace -Zn
'rawfbt::xfs_cfs_cleanup_inode:entry,rawfbt::xfs_cleanup_inode.*:entry {}'
dtrace: description
'rawfbt::xfs_cfs_cleanup_inode:entry,rawfbt::xfs_cleanup_inode.*:entry '
matched 1 probe

So I guess as long as we document this clearly for folks wanting to to
use rawfbt to write portable scripts, it should be okay.

Alan