Messages in this thread | | | Subject | Re: [PATCH v3] Allow user probes on versioned symbols | From | Paul Clarke <> | Date | Wed, 5 Apr 2017 22:45:29 -0500 |
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On 04/04/2017 09:26 AM, Arnaldo Carvalho de Melo wrote: > Em Tue, Apr 04, 2017 at 11:18:02PM +0900, Masami Hiramatsu escreveu: >> On Mon, 3 Apr 2017 11:46:58 -0300 >> Arnaldo Carvalho de Melo <acme@kernel.org> wrote: >> > > > But apart from those problems, I think that one should be able to ask >> > > > for a versioned symbol, to probe just apps using that specific version, >> > >> > > I agree, but wasn't trying to tackle that at the moment. I can look into it, though. >> > >> > > > for instance, we should consider the whole name as two functions, which >> > > > in fact, they are, no? >> > >> > > I'm not sure I understand what you mean here. Do you mean we should set a probe at every version of a given symbol name? For example, if there are symbols: >> > > a@@V2 >> > > a@V1.1 >> > > a@V1 >> > >> > > ...for a request to set a probe at "a", we'd actually set a probe at all 3? >> > >> > I think that we should just probe the default for that symbol and have a >> > way to probe all of them, perhaps using the wildcard, i.e.: >> > >> > [root@jouet linux]# nm /lib64/libpthread-2.24.so | grep ' pthread_cond_timedwait' >> > 000000000000dd90 T pthread_cond_timedwait@GLIBC_2.2.5 >> > 000000000000d6e0 T pthread_cond_timedwait@@GLIBC_2.3.2 >> > [root@jouet linux]# >> > >> > # perf probe -x /lib64/libpthread-2.24.so pthread_cond_timedwait >> > >> > should be equivalent to: >> > >> > # perf probe -x /lib64/libpthread-2.24.so pthread_cond_timedwait@@GLIBC_2.3.2 >> > >> > Which matches how these versioned symbols are resolved by the linker, >> > no? >> > >> > I.e. when 'pthread_cond_timedwait' is specified and the symbol table >> > lookup fails, I think we should re-lookup for >> > 'pthread_cond_timedwait@@*', i.e. we should have a >> > symbol__find_default_by_name(), which will take the >> > "pthread_cond_timedwait" and use a symbol comparison using >> > strncmp(strlen(key)), matching, should then look at right after the >> > common part looking for the double @@. > >> Hm, this 'fallback'process sounds good idea to me.
I just sent a patch that does what you suggest, above. To avoid duplicating the code in symbols_find_by_name, I added a parameter to tell it whether to ignore default symbol tags or not.
> This is just trying to keep the semantics used by the original user of > this syntax, i.e. the linker. > >> BTW, how would we support other SYMBOL@VERSION, since we already >> use '@' for specifying source code? >> One possible way is to support it directly in perf-probe. If it >> failed to find probe point from dwarf, try to find from symbol >> map by using '@VERSION' suffix. > > Right, we would be overloading that @ symbol, since version numbers > usually are very different of file source names :-)
There's not a lot of syntactic difference between "file" and "tag". I don't think there is any standard for what either can be. One might expect a "file" to be name.extension, where extension is a finite set (possibly fairly large). A tag can be almost anything, I think. One might expect it to end with a number. I don't think there's a guarantee of either case, though.
It would seem one way to determine whether it's a file or a tag is to try to find it in the symbol tables. If it's not there, assume it's a file. (Or vice-versa.)
PC
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