Created attachment 1071849 [details]
self-contained reproducer

Description of problem:
ISTM that sometimes hardware watchpoints are not set correctly on armv8.  I have a program and when debugging it I set read watchpoint at 4 byte variable and that triggers sigtrap even when the program reads a variable 4 bytes before that in memory.


Version-Release number of selected component (if applicable):
gdb-7.6.1-78.el7.aarch64
kernel-4.2.0-0.rc6.19.el7.aarch64


How reproducible:
always


Steps to Reproduce:
0. fetch reproducer from bugzilla
1. bash hw_watchpoints.sh

I set a read watchpoint on r on 0x420024, but that triggers sigtrap when reading d from 0x420020.  It does now work if the addresses are reversed.  It also disappears when I read from 0x420028.

By looking at the disassembly it looks to me like the program is doing 32 bit aligned load/stores.  And it's not loading from 0x420024.

9         r=d;
   0x00000000004005e0 <+48>:    adrp    x0, 0x420000 <__libc_start_main>
   0x00000000004005e4 <+52>:    add     x0, x0, #0x20
   0x00000000004005e8 <+56>:    ldr     w1, [x0]
   0x00000000004005ec <+60>:    adrp    x0, 0x420000 <__libc_start_main>
   0x00000000004005f0 <+64>:    add     x0, x0, #0x24
   0x00000000004005f4 <+68>:    str     w1, [x0]



Actual results:
---------------------------------------------
Temporary breakpoint 1 at 0x4005b0: file aarch64.c, line 5.

Temporary breakpoint 1, main () at aarch64.c:5
5         p=1;
Hardware read watchpoint 2: r
$1 = (int *) 0x420028 <p>
$2 = (int *) 0x420024 <r>
$3 = (int *) 0x420020 <d>
9         r=d;

Program received signal SIGTRAP, Trace/breakpoint trap.
0x00000000004005ec in main () at aarch64.c:9
9         r=d;
---------------------------------------------


Expected results:
---------------------------------------------
Temporary breakpoint 1 at 0x4005b0: file aarch64.c, line 5.

Temporary breakpoint 1, main () at aarch64.c:5
5         p=1;
Hardware read watchpoint 2: r
$1 = (int *) 0x420028 <p>
$2 = (int *) 0x420024 <r>
$3 = (int *) 0x420020 <d>
9         r=d;
[Inferior 1 (process 19894) exited normally]
---------------------------------------------