Messages in this thread | | | Date | Tue, 3 Mar 2015 22:31:24 +0900 | From | Namhyung Kim <> | Subject | Re: [PATCH perf/core 2/4] perf-probe: Fix to handle aliased symbols in glibc |
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Hi Masami,
On Mon, Mar 02, 2015 at 09:49:53PM +0900, Masami Hiramatsu wrote: > Fix perf probe to handle aliased symbols correctly in glibc. > In the glibc, several symbols are defined as an alias of > __libc_XXX, e.g. malloc is an alias of __libc_malloc. > In such cases, dwarf has no subroutine instances of the > alias functions (e.g. no "malloc" instance), but the map > has that symbol and its address. > Thus, if we search the alieased symbol in debuginfo, we > always fail to find it, but it is in the map. > > To solve this problem, this fails back to address-based > alternative search, which searches the symbol in the map, > translates its address to alternative (correct) function > name by using debuginfo, and retry to find the alternative > function point from debuginfo. > > This adds fail-back process to --vars, --lines and --add > options. So, now you can use those on malloc@libc :)
So this is only for binaries that have debuginfo, right?
I have a similar issue with no debuginfo.
$ perf probe -x /usr/lib/libc.so.6 -V calloc The /usr/lib/libc-2.21.so file has no debug information. Rebuild with -g, or install an appropriate debuginfo package. Error: Failed to show vars.
But it also failed to add a probe to calloc:
$ perf probe -x /usr/lib/libc.so.6 -a calloc Failed to find symbol calloc in /usr/lib/libc-2.21.so Error: Failed to add events.
Of course there's calloc in the libc binary.
$ nm /usr/lib/libc.so.6 | grep calloc 000000000007b1f0 t __calloc 000000000007b1f0 T __libc_calloc 000000000007b1f0 W calloc
I think the problem is that calloc is a weak symbol so it'll be discarded during the symbol loading. It's because to avoid multiple symbols (or aliases) at a same address so we choose a better symbol using heuristics. But for this case I think we can allow aliases since it's used only for finding probe points.
Thanks, Namhyung
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