[Bug gdb/19487] Result of call to __errno_location() is truncated to 32 bit on amd64

["cvs-commit at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19487

--- Comment #6 from cvs-commit at gcc dot gnu.org <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Pedro Alves <palves@sourceware.org>:

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=bf223d3e808e6fec9ee165d3d48beb74837796de

commit bf223d3e808e6fec9ee165d3d48beb74837796de
Author: Pedro Alves <palves@redhat.com>
Date:   Mon Aug 21 11:34:32 2017 +0100

    Handle function aliases better (PR gdb/19487, errno printing)

    (Ref: https://sourceware.org/ml/gdb/2017-06/msg00048.html)

    This patch improves GDB support for function aliases defined with
    __attribute__ alias.  For example, in the test added by this commit,
    there is no reference to "func_alias" in the debug info at all, only
    to "func"'s definition:

     $ nm  ./testsuite/outputs/gdb.base/symbol-alias/symbol-alias  | grep "
func"
     00000000004005ae t func
     00000000004005ae T func_alias

     $ readelf -w ./testsuite/outputs/gdb.base/symbol-alias/symbol-alias | grep
func -B 1 -A 8
     <1><db>: Abbrev Number: 5 (DW_TAG_subprogram)
        <dc>   DW_AT_name        : (indirect string, offset: 0x111): func
        <e0>   DW_AT_decl_file   : 1
        <e1>   DW_AT_decl_line   : 27
        <e2>   DW_AT_prototyped  : 1
        <e2>   DW_AT_type        : <0xf8>
        <e6>   DW_AT_low_pc      : 0x4005ae
        <ee>   DW_AT_high_pc     : 0xb
        <f6>   DW_AT_frame_base  : 1 byte block: 9c        
(DW_OP_call_frame_cfa)
        <f8>   DW_AT_GNU_all_call_sites: 1

    So all GDB knows about "func_alias" is from the minsym (elf symbol):

     (gdb) p func_alias
     $1 = {<text variable, no debug info>} 0x4005ae <func>
     (gdb) ptype func_alias
     type = int ()

     (gdb) p func
     $2 = {struct S *(void)} 0x4005ae <func>
     (gdb) ptype func
     type = struct S {
         int field1;
         int field2;
     } *(void)

    The result is that calling func_alias from the command line produces
    incorrect results.

    This is similar (though not exactly the same) to the glibc
    errno/__errno_location/__GI___errno_location situation.  On glibc,
    errno is defined like this:

      extern int *__errno_location (void);
      #define errno (*__errno_location ())

    with __GI___errno_location being an internal alias for
    __errno_location.  On my system's libc (F23), I do see debug info for
    __errno_location, in the form of name vs linkage name:

     <1><95a5>: Abbrev Number: 18 (DW_TAG_subprogram)
        <95a6>   DW_AT_external    : 1
        <95a6>   DW_AT_name        : (indirect string, offset: 0x2c26):
__errno_location
        <95aa>   DW_AT_decl_file   : 1
        <95ab>   DW_AT_decl_line   : 24
        <95ac>   DW_AT_linkage_name: (indirect string, offset: 0x2c21):
__GI___errno_location
        <95b0>   DW_AT_prototyped  : 1
        <95b0>   DW_AT_type        : <0x9206>
        <95b4>   DW_AT_low_pc      : 0x20f40
        <95bc>   DW_AT_high_pc     : 0x11
        <95c4>   DW_AT_frame_base  : 1 byte block: 9c      
(DW_OP_call_frame_cfa)
        <95c6>   DW_AT_GNU_all_call_sites: 1

    however that doesn't matter in practice, because GDB doesn't record
    demangled names anyway, and so we end up with the exact same situation
    covered by the testcase.

    So the fix is to make the expression parser find a debug symbol for
    the same address as the just-found minsym, when a lookup by name
    didn't find a debug symbol by name.  We now get:

     (gdb) p func_alias
     $1 = {struct S *(void)} 0x4005ae <func>
     (gdb) p __errno_location
     $2 = {int *(void)} 0x7ffff6e92830 <__errno_location>

    I've made the test exercise variable aliases too, for completeness.
    Those already work correctly, because unlike for function aliases, GCC
    emits debug information for variable aliases.

    Tested on GNU/Linux.

    gdb/ChangeLog:
    2017-08-21  Pedro Alves  <palves@redhat.com>

        PR gdb/19487
        * c-exp.y (variable production): Handle function aliases.
        * minsyms.c (msymbol_is_text): New function.
        * minsyms.h (msymbol_is_text): Declare.
        * symtab.c (find_function_alias_target): New function.
        * symtab.h (find_function_alias_target): Declare.

    gdb/testsuite/ChangeLog:
    2017-08-21  Pedro Alves  <palves@redhat.com>

        PR gdb/19487
        * gdb.base/symbol-alias.c: New.
        * gdb.base/symbol-alias2.c: New.
        * gdb.base/symbol-alias.exp: New.

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