From 872cec9f53482429fbd547e13ac552d472bd002b Mon Sep 17 00:00:00 2001 From: Matthew Malcomson Date: Sun, 28 Jun 2020 13:06:36 +0100 Subject: [PATCH] [Patch] GDB: aarch64: Add ability to displaced step over a BR/BLR instruction Enable displaced stepping over a BR/BLR instruction Displaced stepping over an instruction executes a instruction in a scratch area and then manually fixes up the PC address to leave execution where it would have been if the instruction were in its original location. The BR instruction does not need modification in order to run correctly at a different address, but the displaced step fixup method should not manually adjust the PC since the BR instruction sets that value already. The BLR instruction should also avoid such a fixup, but must also have the link register modified to point to just after the original code location rather than back to the scratch location. This patch adds the above functionality. We add this functionality by modifying aarch64_displaced_step_others rather than by adding a new visitor method to aarch64_insn_visitor. We choose this since it seems that visitor approach is designed specifically for PC relative instructions (which must always be modified when executed in a different location). It seems that the BR and BLR instructions are more like the RET instruction which is already handled specially in aarch64_displaced_step_others. This also means the gdbserver code to relocate an instruction when creating a fast tracepoint does not need to be modified, since nothing special is needed for the BR and BLR instructions there. Manually tested on AArch64 and ensured new testcase passes. Regression tests showed nothing untoward. ------##### Observed (mis)behaviour before was that displaced stepping over a BR or BLR instruction would not execute the function they called. Most easily seen by putting a breakpoint with a condition on such an instruction and a print statement in the functions they called. When run with the breakpoint enabled the function is not called and "numargs called" is not printed. When run with the breakpoint disabled the function is called and the message is printed. --- GDB Session hw-a20-10:gcc-source [15:57:14] % gdb ../using-blr Reading symbols from ../using-blr...done. (gdb) disassemble blr_call_value Dump of assembler code for function blr_call_value: ... 0x0000000000400560 <+28>: blr x2 ... 0x00000000004005b8 <+116>: ret End of assembler dump. (gdb) break *0x0000000000400560 Breakpoint 1 at 0x400560: file ../using-blr.c, line 22. (gdb) condition 1 10 == 0 (gdb) run Starting program: /home/matmal01/using-blr [Inferior 1 (process 33279) exited with code 012] (gdb) disable 1 (gdb) run Starting program: /home/matmal01/using-blr numargs called [Inferior 1 (process 33289) exited with code 012] (gdb) Test program: ---- using-blr ---- \#include typedef int (foo) (int, int); typedef void (bar) (int, int); struct sls_testclass { foo *x; bar *y; int left; int right; }; __attribute__ ((noinline)) int blr_call_value (struct sls_testclass x) { int retval = x.x(x.left, x.right); if (retval % 10) return 100; return 9; } __attribute__ ((noinline)) int blr_call (struct sls_testclass x) { x.y(x.left, x.right); if (x.left % 10) return 100; return 9; } int numargs (__attribute__ ((unused)) int left, __attribute__ ((unused)) int right) { printf("numargs called\n"); return 10; } void altfunc (__attribute__ ((unused)) int left, __attribute__ ((unused)) int right) { printf("altfunc called\n"); } int main(int argc, char **argv) { struct sls_testclass x = { .x = numargs, .y = altfunc, .left = 1, .right = 2 }; if (argc > 2) { blr_call (x); } else blr_call_value (x); return 10; } ------ gdb/ChangeLog: 2020-07-23 Matthew Malcomson * aarch64-tdep.c (aarch64_displaced_step_others): Account for BLR and BR instructions. * arch/aarch64-insn.h (enum aarch64_opcodes): Add BR opcode. (enum aarch64_masks): New. gdb/testsuite/ChangeLog: 2020-07-23 Matthew Malcomson * gdb.arch/aarch64-disp-stepping.exp: New test runner. * gdb.arch/aarch64-disp-stepping.s: New test. --- gdb/aarch64-tdep.c | 19 ++-- gdb/arch/aarch64-insn.h | 10 ++ gdb/testsuite/gdb.arch/aarch64-disp-stepping.exp | 65 +++++++++++++ gdb/testsuite/gdb.arch/aarch64-disp-stepping.s | 111 +++++++++++++++++++++++ 4 files changed, 199 insertions(+), 6 deletions(-) create mode 100644 gdb/testsuite/gdb.arch/aarch64-disp-stepping.exp create mode 100644 gdb/testsuite/gdb.arch/aarch64-disp-stepping.s diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c index 5e7d0d0..d247108 100644 --- a/gdb/aarch64-tdep.c +++ b/gdb/aarch64-tdep.c @@ -2974,15 +2974,22 @@ aarch64_displaced_step_others (const uint32_t insn, struct aarch64_displaced_step_data *dsd = (struct aarch64_displaced_step_data *) data; - aarch64_emit_insn (dsd->insn_buf, insn); - dsd->insn_count = 1; - - if ((insn & 0xfffffc1f) == 0xd65f0000) + uint32_t masked_insn = (insn & CLEAR_Rn_MASK); + if (masked_insn == BLR) { - /* RET */ - dsd->dsc->pc_adjust = 0; + /* Emit a BR to the same register and then update LR to the original + address (similar to aarch64_displaced_step_b). */ + aarch64_emit_insn (dsd->insn_buf, insn & 0xffdfffff); + regcache_cooked_write_unsigned (dsd->regs, AARCH64_LR_REGNUM, + data->insn_addr + 4); } else + aarch64_emit_insn (dsd->insn_buf, insn); + dsd->insn_count = 1; + + if (masked_insn == RET || masked_insn == BR || masked_insn == BLR) + dsd->dsc->pc_adjust = 0; + else dsd->dsc->pc_adjust = 4; } diff --git a/gdb/arch/aarch64-insn.h b/gdb/arch/aarch64-insn.h index 6a63ce9..f261363 100644 --- a/gdb/arch/aarch64-insn.h +++ b/gdb/arch/aarch64-insn.h @@ -40,7 +40,9 @@ enum aarch64_opcodes CBNZ = 0x21000000 | B, TBZ = 0x36000000 | B, TBNZ = 0x37000000 | B, + /* BR 1101 0110 0001 1111 0000 00rr rrr0 0000 */ /* BLR 1101 0110 0011 1111 0000 00rr rrr0 0000 */ + BR = 0xd61f0000, BLR = 0xd63f0000, /* RET 1101 0110 0101 1111 0000 00rr rrr0 0000 */ RET = 0xd65f0000, @@ -107,6 +109,14 @@ enum aarch64_opcodes NOP = (0 << 5) | HINT, }; +/* List of useful masks. */ +enum aarch64_masks +{ + /* Used for masking out an Rn argument from an opcode. */ + CLEAR_Rn_MASK = 0xfffffc1f, +}; + + /* Representation of a general purpose register of the form xN or wN. This type is used by emitting functions that take registers as operands. */ diff --git a/gdb/testsuite/gdb.arch/aarch64-disp-stepping.exp b/gdb/testsuite/gdb.arch/aarch64-disp-stepping.exp new file mode 100644 index 0000000..54eae61 --- /dev/null +++ b/gdb/testsuite/gdb.arch/aarch64-disp-stepping.exp @@ -0,0 +1,65 @@ +# Copyright 2020 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 . +# +# This file is part of the gdb testsuite. + +# Test displaced stepping over BR and BLR instructions. + +if {![is_aarch64_target]} { + verbose "Skipping ${gdb_test_file_name}." + return +} + +standard_testfile ".s" +if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } { + return -1 +} + +gdb_breakpoint "*blr_teststart" +gdb_breakpoint "*blr_testcheck" +gdb_breakpoint "*br_teststart" +gdb_breakpoint "*br_testcheck" + + +# Test for displaced stepping over the BLR instruction. +gdb_test "run" \ + "Starting program.*Breakpoint $decimal.*" \ + "Run until BLR test start" + +set expected_lr [get_hexadecimal_valueof "\$pc + 4" 0] +gdb_test "print/x \$x0" \ + ".. = 0x0" \ + "Ensure x0 is 0 before BLR test." + +gdb_continue_to_breakpoint "BLR test check" + +gdb_test "print/x \$lr == $expected_lr" \ + ".. = 0x1" \ + "Ensure LR is set to just after BLR." +gdb_test "print/x \$x0" \ + ".. = 0x1" \ + "Ensure x0 is 1 after BLR test." + + +# Test for displaced stepping over the BR instruction. +gdb_continue_to_breakpoint "BR test start" + +gdb_test "print/x \$x0" \ + ".. = 0x0" \ + "Ensure x0 is 0 before BR test." +gdb_continue_to_breakpoint "BR test check" +gdb_test "print/x \$x0" \ + ".. = 0x1" \ + "Ensure x0 is 1 after BR test." diff --git a/gdb/testsuite/gdb.arch/aarch64-disp-stepping.s b/gdb/testsuite/gdb.arch/aarch64-disp-stepping.s new file mode 100644 index 0000000..c7a67a1 --- /dev/null +++ b/gdb/testsuite/gdb.arch/aarch64-disp-stepping.s @@ -0,0 +1,111 @@ +/* Copyright 2020 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 . + + This file is part of the gdb testsuite. + + Test displaced stepping over AArch64 instructions. */ + +// Instructions not yet tested. +// - B +// - BL +// - B.COND +// - CBZ +// - CBNZ +// - TBZ +// - TBNZ +// - ADR +// - ADRP +// - LDR (literal) +// - RET + +// Function testing stepping over BLR instruction. + .text + .align 2 + .global test_blr_stepping + .type test_blr_stepping, %function +test_blr_stepping: + .cfi_startproc + // x2 Stores the old LR. + mov x2,x30 + // x0 is the indicator value to show whether the jump happened. + mov x0, #0 + // Load the jump position into register x1 + movz x1, :abs_g3:.LJUMPPOS + movk x1, :abs_g2_nc:.LJUMPPOS + movk x1, :abs_g1_nc:.LJUMPPOS + movk x1, :abs_g0_nc:.LJUMPPOS +blr_teststart: + blr x1 + b blr_testcheck +.LJUMPPOS: + mov x0, #1 +blr_testcheck: + // Put the old LR value back into the LR register. + // Do this for both successful jump and unsuccessful jump since the LR + // will have changed both times and we want the program to continue + // properly both times. + mov x30, x2 + ret + .cfi_endproc + .size test_blr_stepping, .-test_blr_stepping + + +// Function testing stepping over BR instruction. + .text + .align 2 + .global test_br_stepping + .type test_br_stepping, %function +test_br_stepping: + .cfi_startproc + // x0 is the indicator value to show whether the jump happened. + mov x0, #0 + // Load the jump position into register x1 + movz x1, :abs_g3:.LJUMPPOS2 + movk x1, :abs_g2_nc:.LJUMPPOS2 + movk x1, :abs_g1_nc:.LJUMPPOS2 + movk x1, :abs_g0_nc:.LJUMPPOS2 +br_teststart: + br x1 + b br_testcheck +.LJUMPPOS2: + mov x0, #1 +br_testcheck: + ret + .cfi_endproc + .size test_br_stepping, .-test_br_stepping + + + +// Main function calling all test functions above. + .text + .align 2 + .global main + .type main, %function +main: + .cfi_startproc + stp x29, x30, [sp, -16]! + .cfi_def_cfa_offset 16 + .cfi_offset 29, -16 + .cfi_offset 30, -8 + bl test_blr_stepping + bl test_br_stepping + ldp x29, x30, [sp], 16 + .cfi_restore 30 + .cfi_restore 29 + .cfi_def_cfa_offset 0 + ret + .cfi_endproc + .size main, .-main + .section .note.GNU-stack,"",@progbits -- 2.7.4