Re: [PATCH 3/3] perf record: Fix s390 missing module symbol and warning for non-root users

Em Wed, May 22, 2019 at 04:46:01PM +0200, Thomas Richter escreveu:

Command 'perf record' and 'perf report' on a system without kernel
debuginfo packages uses /proc/kallsyms and /proc/modules to find
addresses for kernel and module symbols. On x86 this works for root
and non-root users.

On s390, when invoked as non-root user, many of the following warnings
are shown and module symbols are missing:

    proc/{kallsyms,modules} inconsistency while looking for
        "[sha1_s390]" module!

Command 'perf record' creates a list of module start addresses by
parsing the output of /proc/modules and creates a PERF_RECORD_MMAP
record for the kernel and each module. The following function call
sequence is executed:

  machine__create_kernel_maps
    machine__create_module
      modules__parse
        machine__create_module --> for each line in /proc/modules
          arch__fix_module_text_start

Function arch__fix_module_text_start() is s390 specific. It opens
file /sys/module/<name>/sections/.text to extract the module's .text
section start address. On s390 the module loader prepends a header
before the first section, whereas on x86 the module's text section
address is identical the the module's load address.

However module section files are root readable only. For non-root the
read operation fails and machine__create_module() returns an error.
Command perf record does not generate any PERF_RECORD_MMAP record
for loaded modules. Later command perf report complains about missing
module maps.

To fix this function arch__fix_module_text_start() always returns
success. For root users there is no change, for non-root users
the module's load address is used as module's text start address
(the prepended header then counts as part of the text section).

Thanks, applied.

- Arnaldo