[Bug libc/20478] New: libc_ifunc macro and similar usages leads to false debug-information.

[stli at linux dot vnet.ibm.com]

https://sourceware.org/bugzilla/show_bug.cgi?id=20478

            Bug ID: 20478
           Summary: libc_ifunc macro and similar usages leads to false
                    debug-information.
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: stli at linux dot vnet.ibm.com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

The current s390 ifunc resolver for vector optimized functions and the common
libc_ifunc macro in include/libc-symbols.h uses something like that to define
an ifunc'ed symbol:
extern void *__resolve___strlen(unsigned long int dl_hwcap) asm (strlen);
asm (".type strlen, %gnu_indirect_function");

This leads to false debug information:
objdump --dwarf=info libc.so:
...
<1><1e6424>: Abbrev Number: 43 (DW_TAG_subprogram)
    <1e6425>   DW_AT_external    : 1
    <1e6425>   DW_AT_name        : (indirect string, offset: 0x1146e):
__resolve___strlen
    <1e6429>   DW_AT_decl_file   : 1
    <1e642a>   DW_AT_decl_line   : 23
    <1e642b>   DW_AT_linkage_name: (indirect string, offset: 0x1147a): strlen
    <1e642f>   DW_AT_prototyped  : 1
    <1e642f>   DW_AT_type        : <0x1e4ccd>
    <1e6433>   DW_AT_low_pc      : 0x998e0
    <1e643b>   DW_AT_high_pc     : 0x16
    <1e6443>   DW_AT_frame_base  : 1 byte block: 9c     (DW_OP_call_frame_cfa)
    <1e6445>   DW_AT_GNU_all_call_sites: 1
    <1e6445>   DW_AT_sibling     : <0x1e6459>
 <2><1e6449>: Abbrev Number: 44 (DW_TAG_formal_parameter)
    <1e644a>   DW_AT_name        : (indirect string, offset: 0x1845): dl_hwcap
    <1e644e>   DW_AT_decl_file   : 1
    <1e644f>   DW_AT_decl_line   : 23
    <1e6450>   DW_AT_type        : <0x1e4c8d>
    <1e6454>   DW_AT_location    : 0x122115 (location list)
...

The debuginfo for the ifunc-resolver function contains the DW_AT_linkage_name
field, which names the real function name "strlen". If you perform an inferior
function call to strlen in lldb, then it fails due to something like that:
"error: no matching function for call to 'strlen'
candidate function not viable: no known conversion from 'const char [6]'
to 'unsigned long' for 1st argument"

The unsigned long is the dl_hwcap argument of the resolver function.
The strlen function itself has no debuginfo.

Please have a look at the patchset "[PATCH v2 1/9] Add configure check to test
if gcc supports attribute ifunc."
(https://www.sourceware.org/ml/libc-alpha/2016-08/msg00258.html) on the
mailinglist.

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