[binutils-gdb] MIPS/Linux/native: Supply $zero for the !PTRACE_GETREGS case

[Maciej W.Rozycki <macro at sourceware dot org>]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4e6ff0e1b86f736ba20a5034509ffe9f8f05b971

commit 4e6ff0e1b86f736ba20a5034509ffe9f8f05b971
Author: Maciej W. Rozycki <macro@mips.com>
Date:   Tue May 15 16:26:07 2018 +0100

    MIPS/Linux/native: Supply $zero for the !PTRACE_GETREGS case
    
    With native MIPS/Linux targets the $zero register is inaccessible, with
    its supposed context slot provided by the OS occupied by the $restart
    register.  The PTRACE_GETREGS path takes care of it by artificially
    supplying the hardwired contents of $zero in `mips_supply_gregset' or
    `mips64_supply_gregset', as applicable, however the PTRACE_PEEKUSER
    fallback does not, making the register unavailable, e.g.:
    
    (gdb) info registers
             zero       at       v0       v1       a0       a1       a2       a3
    R0    <unavl> 00000001 00000001 d2f1a9fc 00000000 00000000 00417158 00417150
               t0       t1       t2       t3       t4       t5       t6       t7
    R8   00000004 00000000 fffffff8 00000000 00000000 00000000 00000001 00000007
               s0       s1       s2       s3       s4       s5       s6       s7
    R16  00000000 00405e30 00000000 00500000 00000000 0052ec08 00000000 00000000
               t8       t9       k0       k1       gp       sp       s8       ra
    R24  00000000 00417008 00000000 00000000 0041e220 7fff4ce0 7fff4ce0 00405d0c
           status       lo       hi badvaddr    cause       pc
          <unavl> 00441cf1 00000017 00417004 00800024 00405d10
             fcsr      fir  restart
         00800000 00f30000 00000000
    (gdb)
    
    or (under certain circumstances):
    
    (gdb) stepi
    Register 0 is not available
    (gdb)
    
    This is specifically because `mips_linux_register_addr' and
    `mips64_linux_register_addr', both correctly return -1 for
    MIPS_ZERO_REGNUM, and therefore `linux_nat_trad_target::fetch_registers'
    faithfully marks this register as unavailable.
    
    Supply this register artificially then in the PTRACE_PEEKUSER case as
    well, correcting this issue.
    
    	gdb/
    	* mips-linux-nat.c (mips_linux_nat_target::fetch_registers):
    	Supply the MIPS_ZERO_REGNUM register.

Diff:
---
 gdb/ChangeLog        | 5 +++++
 gdb/mips-linux-nat.c | 8 +++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 1d42c30..8035353 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2018-05-15  Maciej W. Rozycki  <macro@mips.com>
 
+	* mips-linux-nat.c (mips_linux_nat_target::fetch_registers):
+	Supply the MIPS_ZERO_REGNUM register.
+
+2018-05-15  Maciej W. Rozycki  <macro@mips.com>
+
 	* mips-tdep.c (mask_address_var): Make variable static.
 
 2018-05-14  Tom Tromey  <tom@tromey.com>
diff --git a/gdb/mips-linux-nat.c b/gdb/mips-linux-nat.c
index 4e0c51b..b1dbb79 100644
--- a/gdb/mips-linux-nat.c
+++ b/gdb/mips-linux-nat.c
@@ -416,7 +416,13 @@ mips_linux_nat_target::fetch_registers (struct regcache *regcache, int regnum)
   /* If we know, or just found out, that PTRACE_GETREGS does not work, fall
      back to PTRACE_PEEKUSER.  */
   if (!have_ptrace_regsets)
-    linux_nat_trad_target::fetch_registers (regcache, regnum);
+    {
+      linux_nat_trad_target::fetch_registers (regcache, regnum);
+
+      /* Fill the inaccessible zero register with zero.  */
+      if (regnum == MIPS_ZERO_REGNUM || regnum == -1)
+	regcache->raw_supply_zeroed (MIPS_ZERO_REGNUM);
+    }
 }
 
 /* Store REGNO (or all registers if REGNO == -1) to the target