Re: [PATCH v2] perf/probe: Search both .eh_frame and .debug_frame sections for probe location

From: Hemant Kumar
Date: Wed Jan 13 2016 - 05:00:52 EST

On 01/12/2016 04:34 PM, åæéå / HIRAMATUïMASAMI wrote:
Hi Hemant,

From: Hemant Kumar [mailto:hemant@xxxxxxxxxxxxxxxxxx]

perf probe through debuginfo__find_probes() in util/probe-finder.c
checks for the functions' frame descriptions in either .eh_frame section
of an ELF or the .debug_frame. The check is based on whether either one
of these sections is present. Depending on distro, toolchain defaults,
architetcutre, build flags, etc., CFI might be found in either .eh_frame
and/or .debug_frame. Sometimes, it may happen that, .eh_frame, even if
present, may not be complete and may miss some descriptions. Therefore,
to be sure, to find the CFI covering an address we will always have to
investigate both if available.
OK, so we'd better check both cfi's.
+/* Find probe points from debuginfo */
+static int debuginfo__find_probes(struct debuginfo *dbg,
+ struct probe_finder *pf)
+ int ret = 0;
+#if _ELFUTILS_PREREQ(0, 142)
+ Elf *elf;
+ GElf_Ehdr ehdr;
+ GElf_Shdr shdr;
+ if (pf->cfi_eh || pf->cfi_dbg)
+ return debuginfo__find_probe_location(dbg, pf);
+ /* Get the call frame information from this dwarf */
+ elf = dwarf_getelf(dbg->dbg);
+ if (elf == NULL)
+ return -EINVAL;
+ if (gelf_getehdr(elf, &ehdr) == NULL)
+ return -EINVAL;
+ if (elf_section_by_name(elf, &ehdr, &shdr, ".eh_frame", NULL) &&
+ shdr.sh_type == SHT_PROGBITS) {
+ pf->cfi_eh = dwarf_getcfi_elf(elf);
+ } else {
+ pf->cfi_dbg = dwarf_getcfi(dbg->dbg);
+ }
Hmm, if you want to check both of those cfi's, don't we have to do below?

if (elf_section_by_name(elf, &ehdr, &shdr, ".eh_frame", NULL) &&
shdr.sh_type == SHT_PROGBITS)
pf->cfi_eh = dwarf_getcfi_elf(elf);

pf->cfi_dbg = dwarf_getcfi(dbg->dbg);

Then, both of pf->cfi_* will be filled (if the elf has ".eh_frame").


Ah, right. Fixed in v3.

Hemant Kumar