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e4439e4346
Consistently supply hardwired $zero as a zeroed register, correcting
issues with the PTRACE_GETREGS path that currently copies the value of
$restart into $zero as illustrated by this program:
$ cat read.c
int
main (void)
{
char buf[1024];
ssize_t size;
size = read (0, buf, sizeof (buf));
return size;
}
$
and this corresponding debug session:
(gdb) break main
Breakpoint 1 at 0x120000970: file read.c, line 9.
(gdb) target remote :2346
Remote debugging using :2346
Reading symbols from .../sysroot/mips-r2-hard/lib64/ld.so.1...done.
0x000000fff7fca5a0 in __start ()
from .../sysroot/mips-r2-hard/lib64/ld.so.1
(gdb) continue
Continuing.
Breakpoint 1, main () at read.c:9
9 size = read (0, buf, sizeof (buf));
(gdb) info registers
zero at v0 v1
R0 0000000000000000 0000000000000001 000000fff7ffe710 0000000000000000
a0 a1 a2 a3
R4 0000000000000001 000000ffffffeb88 000000ffffffeb98 0000000000000000
a4 a5 a6 a7
R8 000000fff7fc8800 000000fff7fc38f0 000000ffffffeb80 2f2f2f2f2f2f2f2f
t0 t1 t2 t3
R12 0000000000000437 0000000000000002 000000fff7ffd000 0000000120000a00
s0 s1 s2 s3
R16 000000fff7fc7068 0000000120000b90 0000000000000000 0000000000000000
s4 s5 s6 s7
R20 0000000000521d88 0000000000522608 0000000000000000 0000000000000000
t8 t9 k0 k1
R24 0000000000000000 0000000120000970 0000000000000000 0000000000000000
gp sp s8 ra
R28 000000fff7fc8800 000000ffffffea50 0000000000000000 000000fff7e4088c
status lo hi badvaddr
0000000000109cf3 0000000000005ea5 0000000000000211 000000fff7eadf00
cause pc
0000000000800024 0000000120000970
fcsr fir restart
00000000 00f30000 0000000000000000
(gdb) continue
Continuing.
^C
Program received signal SIGINT, Interrupt.
0x000000fff7f084ac in __GI___libc_read (fd=0, buf=0xffffffe640, nbytes=1024)
at ../sysdeps/unix/sysv/linux/read.c:27
27 return SYSCALL_CANCEL (read, fd, buf, nbytes);
(gdb) info registers
zero at v0 v1
R0 0000000000001388 0000000000000001 0000000000000200 000000fff7ffe710
a0 a1 a2 a3
R4 0000000000000000 000000ffffffe640 0000000000000400 0000000000000001
a4 a5 a6 a7
R8 000000fff7fc8800 000000fff7fc38f0 000000ffffffeb80 2f2f2f2f2f2f2f2f
t0 t1 t2 t3
R12 00000000000005e3 0000000000000002 000000fff7ffd000 000000012000099c
s0 s1 s2 s3
R16 000000fff7fc7068 0000000120000b90 0000000000000000 0000000000000000
s4 s5 s6 s7
R20 0000000000521d88 0000000000522608 0000000000000000 0000000000000000
t8 t9 k0 k1
R24 0000000000000000 000000fff7f2da20 0000000000000000 0000000000000000
gp sp s8 ra
R28 000000fff7fc8800 000000ffffffe600 0000000000000000 000000012000099c
status lo hi badvaddr
0000000000109cf3 00000000000001e6 00000000000000be 000000fff7f08470
cause pc
0000000000800020 000000fff7f084ac
fcsr fir restart
00000000 00f30000 0000000000001388
(gdb)
and with the PTRACE_PEEKUSR path that does not supply this register at
all, causing issues analogous to ones addressed for the native MIPS
backend with commit 4e6ff0e1b86f ("MIPS/Linux/native: Supply $zero for
the !PTRACE_GETREGS case"):
(gdb) info registers
zero at v0 v1
R0 <unavailable> 0000000000000001 0000000000000001 0000000000000000
a0 a1 a2 a3
R4 00000001200212b0 0000000000000000 0000000000000021 000000012001a260
a4 a5 a6 a7
R8 000000012001a260 0000000000000004 800000010cab1680 fffffffffffffff8
t0 t1 t2 t3
R12 0000000000000000 000000fff7edab68 0000000000000001 0000000000000000
s0 s1 s2 s3
R16 000000fff7ee2068 0000000120008b80 0000000000000000 0000000000000000
s4 s5 s6 s7
R20 000000000052e5c8 000000000052f008 0000000000000000 0000000000000000
t8 t9 k0 k1
R24 0000000000000000 00000001200027c0 0000000000000000 0000000000000000
gp sp s8 ra
R28 00000001200212b0 000000ffffffc880 000000ffffffc880 0000000120005ee8
status lo hi badvaddr
<unavailable> 0000000000943efe 000000000000000e 000000012001a008
cause pc
0000000000800024 0000000120005ee8
fcsr fir restart
0e800000 00f30000 0000000000000000
(gdb)
and (under certain circumstances):
(gdb) next
Register 0 is not available
(gdb)
The problem with PTRACE_GETREGS happens because `mips_store_gregset'
supplies the contents of register slot #0, occupied by $restart, to
$zero. The problem with PTRACE_PEEKUSR happens because for $zero
`mips_cannot_fetch_register' returns one, and no alternative way to
supply that register has been defined.
Correct `mips_store_gregset' then for the PTRACE_GETREGS case and add
`mips_fetch_register' for the PTRACE_PEEKUSR case.
gdb/gdbserver/
* linux-mips-low.c (mips_fetch_register): New function. Update
preceding comment.
(mips_store_gregset): Supply 0 rather than $restart for $zero.
(the_low_target): Wire `mips_fetch_register'.
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