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From: Andrew Burgess <aburgess@redhat.com>
To: gdb-patches@sourceware.org
Subject: Re: [PATCHv2] gdb/c++: fix handling of breakpoints on @plt symbols
Date: Mon, 30 Jan 2023 15:13:15 +0000	[thread overview]
Message-ID: <87fsbsukc4.fsf@redhat.com> (raw)
In-Reply-To: <29e560fd9c94874f0839924fa25557a7e8418ad3.1674215225.git.aburgess@redhat.com>


Ping.

Andrew Burgess <aburgess@redhat.com> writes:

> Changes since v1:
>
>   - Rebased to current HEAD of master,
>
>   - Updated test to use 'require allow_shlib_tests', and wrapped a few
>     long lines.  Also Copyright date ranges have been updated for 2023.
>
> ---
>
> 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, for language C++
> and mode MATCH_PARAMS, I explicitly allow a trailing '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
> ---
>  .../gdb.cp/breakpoint-shlib-func-lib.cc       | 21 +++++
>  gdb/testsuite/gdb.cp/breakpoint-shlib-func.cc | 22 ++++++
>  .../gdb.cp/breakpoint-shlib-func.exp          | 78 +++++++++++++++++++
>  gdb/utils.c                                   | 26 ++++++-
>  4 files changed, 146 insertions(+), 1 deletion(-)
>  create mode 100644 gdb/testsuite/gdb.cp/breakpoint-shlib-func-lib.cc
>  create mode 100644 gdb/testsuite/gdb.cp/breakpoint-shlib-func.cc
>  create mode 100644 gdb/testsuite/gdb.cp/breakpoint-shlib-func.exp
>
> diff --git a/gdb/testsuite/gdb.cp/breakpoint-shlib-func-lib.cc b/gdb/testsuite/gdb.cp/breakpoint-shlib-func-lib.cc
> new file mode 100644
> index 00000000000..7219f7c5a23
> --- /dev/null
> +++ b/gdb/testsuite/gdb.cp/breakpoint-shlib-func-lib.cc
> @@ -0,0 +1,21 @@
> +/* Copyright 2022-2023 Free Software Foundation, Inc.
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +extern int foo ();
> +
> +int
> +foo ()
> +{
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.cp/breakpoint-shlib-func.cc b/gdb/testsuite/gdb.cp/breakpoint-shlib-func.cc
> new file mode 100644
> index 00000000000..a86d06560d4
> --- /dev/null
> +++ b/gdb/testsuite/gdb.cp/breakpoint-shlib-func.cc
> @@ -0,0 +1,22 @@
> +/* Copyright 2022-2023 Free Software Foundation, Inc.
> +   This program is free software; you can redistribute it and/or modify
> +   it under the terms of the GNU General Public License as published by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
> +
> +extern int foo ();
> +
> +int
> +main ()
> +{
> +  int answer = foo ();
> +  return answer;
> +}
> diff --git a/gdb/testsuite/gdb.cp/breakpoint-shlib-func.exp b/gdb/testsuite/gdb.cp/breakpoint-shlib-func.exp
> new file mode 100644
> index 00000000000..5d5c87d0b51
> --- /dev/null
> +++ b/gdb/testsuite/gdb.cp/breakpoint-shlib-func.exp
> @@ -0,0 +1,78 @@
> +# Copyright 2022-2023 Free Software Foundation, Inc.
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +
> +# Places a breakpoint on a function in a shared library before the
> +# inferior has started.  GDB will place the breakpoint on the @plt
> +# symbol in the main executable.
> +#
> +# When the inferior is started GDB will re-evaluate the breakpoint
> +# location and move the breakpoint to the function implementation in
> +# the shared library.
> +#
> +# Then, with the inferior started, delete all breakpoints, and
> +# re-create the breakpoint on the shared library function, GDB should
> +# place a single breakpoint on the function implementation in the
> +# shared library.
> +
> +require allow_shlib_tests
> +
> +standard_testfile .cc -lib.cc
> +
> +set libobj [standard_output_file libfoo.so]
> +if {[build_executable "build shared library" $libobj $srcfile2 \
> +	 {debug c++ shlib}] != 0} {
> +    return -1
> +}
> +
> +if {[prepare_for_testing "failed to prepare" $testfile $srcfile \
> +	 [list debug c++ shlib=$libobj]]} {
> +    return -1
> +}
> +
> +# Place the breakpoint before the shared library has been loaded, the
> +# breakpoint should be placed on the @plt symbol.
> +gdb_test "break foo" "Breakpoint $decimal at $hex"
> +gdb_test "info breakpoints" "<foo\\(\\)@plt>"
> +
> +# This is used as an override for delete_breakpoints when we don't
> +# want functions in gdb.exp to delete breakpoints behind the scenes
> +# for us.
> +proc do_not_delete_breakpoints {} {
> +    # Just do nothing.
> +}
> +
> +# Runto main, but don't delete all the breakpoints.
> +with_override delete_breakpoints do_not_delete_breakpoints {
> +    if {![runto_main]} {
> +	return -1
> +    }
> +}
> +
> +# The breakpoint should now be showing in `foo` for real.
> +gdb_test "info breakpoints" \
> +    "\r\n$decimal\\s+\[^\r\n\]+ in foo\\(\\) at \[^\r\n\]+\r\n.*" \
> +    "check breakpoints after starting the inferior"
> +
> +# Now we can delete the breakpoints.
> +delete_breakpoints
> +
> +# And recreate the foo breakpoint, we should only get one location,
> +# the actual location.
> +gdb_test "break foo" "Breakpoint $decimal at \[^\r\n\]+" \
> +    "recreate foo breakpoint"
> +
> +# Check the breakpoint was recreated correctly.
> +gdb_test "info breakpoints" \
> +    "\r\n$decimal\\s+\[^\r\n\]+ in foo\\(\\) at \[^\r\n\]+" \
> +    "check breakpoints after recreation"
> diff --git a/gdb/utils.c b/gdb/utils.c
> index 734c5bf7f70..e64fec941f1 100644
> --- a/gdb/utils.c
> +++ b/gdb/utils.c
> @@ -2397,7 +2397,31 @@ strncmp_iw_with_mode (const char *string1, const char *string2,
>  	  return 0;
>  	}
>        else
> -	return (*string1 != '\0' && *string1 != '(');
> +	{
> +	  if (*string1 == '(')
> +	    {
> +	      int p_count = 0;
> +
> +	      do
> +		{
> +		  if (*string1 == '(')
> +		    ++p_count;
> +		  else if (*string1 == ')')
> +		    --p_count;
> +		  ++string1;
> +		}
> +	      while (*string1 != '\0' && p_count > 0);
> +
> +	      /* There maybe things like 'const' after the parameters,
> +		 which we do want to ignore.  However, if there's an '@'
> +		 then this likely indicates something like '@plt' which we
> +		 should not ignore.  */
> +	      return *string1 == '@';
> +	    }
> +
> +	  return *string1 == '\0' ? 0 : 1;
> +	}
> +
>      }
>    else
>      return 1;
>
> base-commit: 7d02a94c8f74613edb0cdde11982b22eaaa9affe
> -- 
> 2.25.4


  reply	other threads:[~2023-01-30 15:13 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-20 13:55 [PATCH] " Andrew Burgess
2023-01-20 11:48 ` [PATCHv2] " Andrew Burgess
2023-01-30 15:13   ` Andrew Burgess [this message]
2023-02-08  9:36   ` Bruno Larsen
2023-02-10 19:09     ` Andrew Burgess
2023-02-13  9:31       ` Bruno Larsen
2023-02-08 22:22   ` Simon Marchi
2023-02-10 19:10     ` Andrew Burgess
2023-02-10 19:03   ` [PATCHv3] " Andrew Burgess
2023-02-10 20:51     ` Simon Marchi
2023-02-12  6:22       ` Andrew Burgess
2023-02-13 14:14       ` Tom Tromey

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