Andrew Haley found a bug on GDB running on ARM when using
--enable-64-bit-bfd. Basically the issue happens when dealing with "bl"
instructions: GDB does branch destination calculation and (wrongly)
sign-extends the PC. Here is a piece of his original message explaining
the problem:
> next_pc = arm_get_next_pc (frame, get_frame_pc (frame));
>
> /* The Linux kernel offers some user-mode helpers in a high page. We can
> not read this page (as of 2.6.23), and even if we could then we couldn't
> set breakpoints in it, and even if we could then the atomic operations
> would fail when interrupted. They are all called as functions and return
> to the address in LR, so step to there instead. */
> if (next_pc > 0xffff0000)
> next_pc = get_frame_register_unsigned (frame, ARM_LR_REGNUM);
>
> arm_insert_single_step_breakpoint (gdbarch, aspace, next_pc);
>
> Unfortunately, branch destination addresses are SIGN EXTENDED to 64
> bits. So,
>
> (top-gdb) p/x next_pc
> $14 = 0xffffffffb6df2864
>
> Which triggers the next_pc = get_frame_register_unsigned(), and we
> cannot step into any branches because the destination PC is wrong.
Anyway, the fix is simple and Andrew himself provided it for us. It
took a while for me to figure out how to trigger the bug (in order to
write a testcase for it), but I finally made it.
The attached patch fixes the problem (by casting to `unsigned long'
instead of just `long'), and also includes a testcase to reproduce the
issue.
gdb/ChangeLog:
2013-04-22 Andrew Haley <aph@redhat.com>
* arm-tdep.c (BranchDest): Cast result as "unsigned long",
instead of "long".
gdb/testsuite/ChangeLog:
2013-04-22 Sergio Durigan Junior <sergiodj@redhat.com>
* gdb.arch/arm-bl-branch-dest.c: New file.
* gdb.arch/arm-bl-branch-dest.exp: Likewise.
2013-04-22 09:20:33 +00:00
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# Copyright (C) 2013 Free Software Foundation, Inc.
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#
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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 { ![istarget "arm*-*-*"] } {
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verbose "Skipping ${testfile}."
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return
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}
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standard_testfile
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# We need to load the text segment in a high address. This is because
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# the bug we are dealing with happened when GDB sign-extended the PC
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# on ARM, causing the PC to acquire a wrong value. That's why we use
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# the "-Wl,-Ttext-segment" option compile the binary.
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if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} \
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2013-04-22 09:32:21 +00:00
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[list debug ldflags=-Wl,-Ttext-segment=0xb0000000]] } {
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Andrew Haley found a bug on GDB running on ARM when using
--enable-64-bit-bfd. Basically the issue happens when dealing with "bl"
instructions: GDB does branch destination calculation and (wrongly)
sign-extends the PC. Here is a piece of his original message explaining
the problem:
> next_pc = arm_get_next_pc (frame, get_frame_pc (frame));
>
> /* The Linux kernel offers some user-mode helpers in a high page. We can
> not read this page (as of 2.6.23), and even if we could then we couldn't
> set breakpoints in it, and even if we could then the atomic operations
> would fail when interrupted. They are all called as functions and return
> to the address in LR, so step to there instead. */
> if (next_pc > 0xffff0000)
> next_pc = get_frame_register_unsigned (frame, ARM_LR_REGNUM);
>
> arm_insert_single_step_breakpoint (gdbarch, aspace, next_pc);
>
> Unfortunately, branch destination addresses are SIGN EXTENDED to 64
> bits. So,
>
> (top-gdb) p/x next_pc
> $14 = 0xffffffffb6df2864
>
> Which triggers the next_pc = get_frame_register_unsigned(), and we
> cannot step into any branches because the destination PC is wrong.
Anyway, the fix is simple and Andrew himself provided it for us. It
took a while for me to figure out how to trigger the bug (in order to
write a testcase for it), but I finally made it.
The attached patch fixes the problem (by casting to `unsigned long'
instead of just `long'), and also includes a testcase to reproduce the
issue.
gdb/ChangeLog:
2013-04-22 Andrew Haley <aph@redhat.com>
* arm-tdep.c (BranchDest): Cast result as "unsigned long",
instead of "long".
gdb/testsuite/ChangeLog:
2013-04-22 Sergio Durigan Junior <sergiodj@redhat.com>
* gdb.arch/arm-bl-branch-dest.c: New file.
* gdb.arch/arm-bl-branch-dest.exp: Likewise.
2013-04-22 09:20:33 +00:00
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return -1
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}
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if { ![runto_main] } {
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return -1
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}
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gdb_test "next" "\[0-9\]+\\s+return 0;"
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