gdb/c++: fix handling of breakpoints on @plt symbols

This commit should fix PR gdb/20091, PR gdb/17201, and PR gdb/17071.
Additionally, PR gdb/17199 relates to this area of code, but is more
of a request to refactor some parts of GDB, this commit does not
address that request, but it is probably worth reading that PR when
looking at this commit.

When the current language is C++, and the user places a breakpoint on
a function in a shared library, GDB will currently find two locations
for the breakpoint, one location will be within the function itself as
we would expect, but the other location will be within the PLT table
for the call to the named function.  Consider this session:

  $ gdb -q /tmp/breakpoint-shlib-func
  Reading symbols from /tmp/breakpoint-shlib-func...
  (gdb) start
  Temporary breakpoint 1 at 0x40112e: file /tmp/breakpoint-shlib-func.cc, line 20.
  Starting program: /tmp/breakpoint-shlib-func

  Temporary breakpoint 1, main () at /tmp/breakpoint-shlib-func.cc:20
  20	  int answer = foo ();
  (gdb) break foo
  Breakpoint 2 at 0x401030 (2 locations)
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   <MULTIPLE>
  2.1                         y   0x0000000000401030 <foo()@plt>
  2.2                         y   0x00007ffff7fc50fd in foo() at /tmp/breakpoint-shlib-func-lib.cc:20

This is not the expected behaviour.  If we compile the same test using
a C compiler then we see this:

  (gdb) break foo
  Breakpoint 2 at 0x7ffff7fc50fd: file /tmp/breakpoint-shlib-func-c-lib.c, line 20.
  (gdb) info breakpoints
  Num     Type           Disp Enb Address            What
  2       breakpoint     keep y   0x00007ffff7fc50fd in foo at /tmp/breakpoint-shlib-func-c-lib.c:20

Here's what's happening.  When GDB parses the symbols in the main
executable and the shared library we see a number of different symbols
for foo, and use these to create entries in GDB's msymbol table:

  - In the main executable we see a symbol 'foo@plt' that points at
    the plt entry for foo, from this we add two entries into GDB's
    msymbol table, one called 'foo@plt' which points at the plt entry
    and has type mst_text, then we create a second symbol, this time
    called 'foo' with type mst_solib_trampoline which also points at
    the plt entry,

  - Then, when the shared library is loaded we see another symbol
    called 'foo', this one points at the actual implementation in the
    shared library.  This time GDB creates a msymbol called 'foo' with
    type mst_text that points at the implementation.

This means that GDB creates 3 msymbols to represent the 2 symbols
found in the executable and shared library.

When the user creates a breakpoint on 'foo' GDB eventually ends up in
search_minsyms_for_name (linespec.c), this function then calls
iterate_over_minimal_symbols passing in the name we are looking for
wrapped in a lookup_name_info object.

In iterate_over_minimal_symbols we iterate over two hash tables (using
the name we're looking for as the hash key), first we walk the hash
table of symbol linkage names, then we walk the hash table of
demangled symbol names.

When the language is C++ the symbols for 'foo' will all have been
mangled, as a result, in this case, the iteration of the linkage name
hash table will find no matching results.

However, when we walk the demangled hash table we do find some
results.  In order to match symbol names, GDB obtains a symbol name
matching function by calling the get_symbol_name_matcher method on the
language_defn class.  For C++, in this case, the matching function we
use is cp_fq_symbol_name_matches, which delegates the work to
strncmp_iw_with_mode with mode strncmp_iw_mode::MATCH_PARAMS and
language set to language_cplus.

The strncmp_iw_mode::MATCH_PARAMS mode means that strncmp_iw_mode will
skip any parameters in the demangled symbol name when checking for a
match, e.g. 'foo' will match the demangled name 'foo()'.  The way this
is done is that the strings are matched character by character, but,
once the string we are looking for ('foo' here) is exhausted, if we
are looking at '(' then we consider the match a success.

Lets consider the 3 symbols GDB created.  If the function declaration
is 'void foo ()' then from the main executable we added symbols
'_Z3foov@plt' and '_Z3foov', while from the shared library we added
another symbol call '_Z3foov'.  When these are demangled they become
'foo()@plt', 'foo()', and 'foo()' respectively.

Now, the '_Z3foov' symbol from the main executable has the type
mst_solib_trampoline, and in search_minsyms_for_name, we search for
any symbols of type mst_solib_trampoline and filter these out of the
results.

However, the '_Z3foov@plt' symbol (from the main executable), and the
'_Z3foov' symbol (from the shared library) both have type mst_text.

During the demangled name matching, due to the use of MATCH_PARAMS
mode, we stop the comparison as soon as we hit a '(' in the demangled
name.  And so, '_Z3foov@plt', which demangles to 'foo()@plt' matches
'foo', and '_Z3foov', which demangles to 'foo()' also matches 'foo'.

By contrast, for C, there are no demangled hash table entries to be
iterated over (in iterate_over_minimal_symbols), we only consider the
linkage name symbols which are 'foo@plt' and 'foo'.  The plain 'foo'
symbol obviously matches when we are looking for 'foo', but in this
case the 'foo@plt' will not match due to the '@plt' suffix.

And so, when the user asks for a breakpoint in 'foo', and the language
is C, search_minsyms_for_name, returns a single msymbol, the mst_text
symbol for foo in the shared library, while, when the language is C++,
we get two results, '_Z3foov' for the shared library function, and
'_Z3foov@plt' for the plt entry in the main executable.

I propose to fix this in strncmp_iw_with_mode.  When the mode is
MATCH_PARAMS, instead of stopping at a '(' and assuming the match is a
success, GDB will instead search forward for the matching, closing,
')', effectively skipping the parameter list, and then resume
matching.  Thus, when comparing 'foo' to 'foo()@plt' GDB will
effectively compare against 'foo@plt' (skipping the parameter list),
and the match will fail, just as it does when the language is C.

There is one slight complication, which is revealed by the test
gdb.linespec/cpcompletion.exp, when searching for the symbol of a
const member function, the demangled symbol will have 'const' at the
end of its name, e.g.:

  struct_with_const_overload::const_overload_fn() const

Previously, the matching would stop at the '(' character, but after my
change the whole '()' is skipped, and the match resumes.  As a result,
the 'const' modifier results in a failure to match, when previously
GDB would have found a match.

To work around this issue, in strncmp_iw_with_mode, when mode is
MATCH_PARAMS, after skipping the parameter list, if the next character
is '@' then we assume we are looking at something like '@plt' and
return a value indicating the match failed, otherwise, we return a
value indicating the match succeeded, this allows things like 'const'
to be skipped.

With these changes in place I now see GDB correctly setting a
breakpoint only at the implementation of 'foo' in the shared library.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20091
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17201
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17071
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17199

Tested-By: Bruno Larsen <blarsen@redhat.com>
Approved-By: Simon Marchi <simon.marchi@efficios.com>

0 files changed, 0 insertions, 0 deletions