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[amd64] Invalid return address after displaced stepping
Making all-stop run on top of non-stop caused a small regression in behavior. This was observed on x86_64-linux. The attached testcase is in C whereas the investigation was done with an Ada program, but it's the same scenario, and using a C testcase allows wider testing. Basically: I am debugging a single-threaded program, and currently stopped inside a function provided by a shared-library, at a line calling a subprogram provided by a second shared library, and trying to "next" over that function call. Before we changed the default all-stop behavior, we had: 7 Impl_Initialize; -- Stop here and try "next" over this line (gdb) n 8 return 5; <<-- OK But now, "next" just stops much earlier: (gdb) n 0x00007ffff7bd8560 in impl.initialize@plt () from /[...]/lib/libpck.so What happens is that next stops at a call instruction, which calls the function's PLT, and GDB fails to notice that the inferior stepped into a subroutine, and so decides that we're done. We can see another symptom of the same issue by looking at the backtrace at the point GDB stopped: (gdb) bt #0 0x00007ffff7bd8560 in impl.initialize@plt () from /[...]/lib/libpck.so #1 0x00000000f7bd86f9 in ?? () #2 0x00007fffffffdf50 in ?? () #3 0x0000000000401893 in a () at /[...]/a.adb:7 Backtrace stopped: frame did not save the PC With a functioning GDB, the backtrace looks like the following instead: #0 0x00007ffff7bd8560 in impl.initialize@plt () from /[...]/lib/libpck.so #1 0x00007ffff7bd86f9 in sub () at /[...]/pck.adb:7 #2 0x0000000000401893 in a () at /[...]/a.adb:7 Note how, for frame #1, the address looks quite similar, except for the high-order bits not being set: #1 0x00007ffff7bd86f9 in sub () at /[...]/pck.adb:7 <<<-- OK #1 0x00000000f7bd86f9 in ?? () <<<-- WRONG ^^^^ |||| Wrong Investigating this further led me to displaced stepping. As we are "next"-ing from a location where a breakpoint is inserted, we need to step out of it, and since we're on non-stop mode, we need to do it using displaced stepping. And looking at amd64-tdep.c:amd64_displaced_step_fixup, I found the code that handles the return address: regcache_cooked_read_unsigned (regs, AMD64_RSP_REGNUM, &rsp); retaddr = read_memory_unsigned_integer (rsp, retaddr_len, byte_order); retaddr = (retaddr - insn_offset) & 0xffffffffUL; The mask used to compute retaddr looks wrong to me, keeping only 4 bytes instead of 8, and explains why the high order bits of the backtrace are unset. What happens is that, after the displaced stepping has completed, GDB restores that return address at the location where the program expects it. But because the top half bits of the address have been masked out, the return address is now invalid. The incorrect behavior of the "next" command and the backtrace at that location are the first symptoms of that. Another symptom is that this actually alters the behavior of the program, where a "cont" from there soon leads to a SEGV when the inferior tries to jump back to that incorrect return address: (gdb) c Continuing. Program received signal SIGSEGV, Segmentation fault. 0x00000000f7bd86f9 in ?? () ^^^^^^^^^^^^^^^^^^ This patch fixes the issue by using a mask that seems more appropriate for this architecture. gdb/ChangeLog: * amd64-tdep.c (amd64_displaced_step_fixup): Fix the mask used to compute RETADDR. gdb/testsuite/ChangeLog: * gdb.base/dso2dso-dso2.c, gdb.base/dso2dso-dso2.h, gdb.base/dso2dso-dso1.c, gdb.base/dso2dso-dso1.h, gdb.base/dso2dso.c, gdb.base/dso2dso.exp: New files. Tested on x86_64-linux, no regression.
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‎gdb/ChangeLog

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2015-08-13 Joel Brobecker <[email protected]>
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* amd64-tdep.c (amd64_displaced_step_fixup): Fix the mask used to
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compute RETADDR.
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2015-08-12 Luis Machado <[email protected]>
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* breakpoint.c (bp_loc_is_permanent): Return 0 when breakpoint

‎gdb/amd64-tdep.c

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regcache_cooked_read_unsigned (regs, AMD64_RSP_REGNUM, &rsp);
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retaddr = read_memory_unsigned_integer (rsp, retaddr_len, byte_order);
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retaddr = (retaddr - insn_offset) & 0xffffffffUL;
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retaddr = (retaddr - insn_offset) & 0xffffffffffffffffULL;
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write_memory_unsigned_integer (rsp, retaddr_len, byte_order, retaddr);
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if (debug_displaced)

‎gdb/testsuite/ChangeLog

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2015-08-13 Joel Brobecker <[email protected]>
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* gdb.base/dso2dso-dso2.c, gdb.base/dso2dso-dso2.h,
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gdb.base/dso2dso-dso1.c, gdb.base/dso2dso-dso1.h, gdb.base/dso2dso.c,
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gdb.base/dso2dso.exp: New files.
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2015-08-10 Doug Evans <[email protected]>
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Keith Seitz <[email protected]>
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‎gdb/testsuite/gdb.base/dso2dso-dso1.c

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/* Copyright 2015 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include "dso2dso-dso1.h"
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#include "dso2dso-dso2.h"
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int
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sub1 (void)
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{
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sub2 (); /* STOP HERE. */
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return 5;
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}

‎gdb/testsuite/gdb.base/dso2dso-dso1.h

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/* Copyright 2015 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef DSO2DSO_DSO1_H
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#define DSO2DSO_DSO1_H
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extern int sub1 (void);
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#endif

‎gdb/testsuite/gdb.base/dso2dso-dso2.c

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/* Copyright 2015 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include "dso2dso-dso2.h"
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void
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sub2 (void)
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{
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/* Do nothing. */
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}

‎gdb/testsuite/gdb.base/dso2dso-dso2.h

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/* Copyright 2015 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#ifndef DSO2DSO_DSO2_H
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#define DSO2DSO_DSO2_H
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extern void sub2 (void);
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#endif

‎gdb/testsuite/gdb.base/dso2dso.c

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/* Copyright 2015 Free Software Foundation, Inc.
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This file is part of GDB.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <http://www.gnu.org/licenses/>. */
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#include "dso2dso-dso1.h"
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int
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main (void)
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{
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int ignored = sub1 ();
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return 0;
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}

‎gdb/testsuite/gdb.base/dso2dso.exp

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# Copyright 2015 Free Software Foundation, Inc.
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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if { [skip_shlib_tests] } {
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return 0
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}
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standard_testfile
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set output_dir [standard_output_file {}]
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set libdso2 $testfile-dso2
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set srcfile_libdso2 $srcdir/$subdir/$libdso2.c
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set binfile_libdso2 [standard_output_file $libdso2.so]
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set libdso1 $testfile-dso1
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set srcfile_libdso1 $srcdir/$subdir/$libdso1.c
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set binfile_libdso1 [standard_output_file $libdso1.so]
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if { [gdb_compile_shlib $srcfile_libdso2 $binfile_libdso2 \
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[list debug additional_flags=-fPIC]] != "" } {
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untested "Could not compile $binfile_libdso2."
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return -1
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}
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if { [gdb_compile_shlib $srcfile_libdso1 $binfile_libdso1 \
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[list debug additional_flags=-fPIC]] != "" } {
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untested "Could not compile $binfile_libdso1."
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return -1
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}
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if { [gdb_compile $srcdir/$subdir/$srcfile $binfile executable \
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[list debug shlib=$binfile_libdso2 shlib=$binfile_libdso1]] != "" } {
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return -1
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}
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clean_restart $binfile
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gdb_load_shlibs $binfile_libdso2 $binfile_libdso1
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if { ![runto_main] } {
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return -1
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}
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# Verify that we can "next" over the call to sub2 (provided by
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# libdso2) make from sub1 (provided by libdso1), and land at
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# the expected location.
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set bp_location [gdb_get_line_number "STOP HERE" $srcfile_libdso1]
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gdb_breakpoint ${srcfile_libdso1}:${bp_location}
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gdb_continue_to_breakpoint "at call to sub2" \
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".*sub2 ().*"
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gdb_test "next" \
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".*return 5;.*" \
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"next over call to sub2"

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