X-Git-Url: http://git.efficios.com/?a=blobdiff_plain;f=gdb%2Famd64-linux-nat.c;h=8d0e8eb35cd2bd3af599cae48f888044d3393663;hb=422186a95cbb02ee1d34388adceeb4bd289074b9;hp=a0bb105f5aa88a341dfbb1cd900d9b65a7364400;hpb=4de283e4b5f21207fe12f99913d1f28d4f07843c;p=deliverable%2Fbinutils-gdb.git

diff --git a/gdb/amd64-linux-nat.c b/gdb/amd64-linux-nat.c
index a0bb105f5a..8d0e8eb35c 100644
--- a/gdb/amd64-linux-nat.c
+++ b/gdb/amd64-linux-nat.c
@@ -92,6 +92,71 @@ static int amd64_linux_gregset32_reg_offset[] =
 /* Transfering the general-purpose registers between GDB, inferiors
    and core files.  */
 
+/* See amd64_collect_native_gregset.  This linux specific version handles
+   issues with negative EAX values not being restored correctly upon syscall
+   return when debugging 32-bit targets.  It has no effect on 64-bit
+   targets.  */
+
+static void
+amd64_linux_collect_native_gregset (const struct regcache *regcache,
+			            void *gregs, int regnum)
+{
+  amd64_collect_native_gregset (regcache, gregs, regnum);
+
+  struct gdbarch *gdbarch = regcache->arch ();
+  if (gdbarch_bfd_arch_info (gdbarch)->bits_per_word == 32)
+    {
+      /* Sign extend EAX value to avoid potential syscall restart
+	 problems.  
+
+	 On Linux, when a syscall is interrupted by a signal, the
+	 (kernel function implementing the) syscall may return
+	 -ERESTARTSYS when a signal occurs.  Doing so indicates that
+	 the syscall is restartable.  Then, depending on settings
+	 associated with the signal handler, and after the signal
+	 handler is called, the kernel can then either return -EINTR
+	 or it can cause the syscall to be restarted.  We are
+	 concerned with the latter case here.
+	 
+	 On (32-bit) i386, the status (-ERESTARTSYS) is placed in the
+	 EAX register.  When debugging a 32-bit process from a 64-bit
+	 (amd64) GDB, the debugger fetches 64-bit registers even
+	 though the process being debugged is only 32-bit.  The
+	 register cache is only 32 bits wide though; GDB discards the
+	 high 32 bits when placing 64-bit values in the 32-bit
+	 regcache.  Normally, this is not a problem since the 32-bit
+	 process should only care about the lower 32-bit portions of
+	 these registers.  That said, it can happen that the 64-bit
+	 value being restored will be different from the 64-bit value
+	 that was originally retrieved from the kernel.  The one place
+	 (that we know of) where it does matter is in the kernel's
+	 syscall restart code.  The kernel's code for restarting a
+	 syscall after a signal expects to see a negative value
+	 (specifically -ERESTARTSYS) in the 64-bit RAX register in
+	 order to correctly cause a syscall to be restarted.
+	 
+	 The call to amd64_collect_native_gregset, above, is setting
+	 the high 32 bits of RAX (and other registers too) to 0.  For
+	 syscall restart, we need to sign extend EAX so that RAX will
+	 appear as a negative value when EAX is set to -ERESTARTSYS. 
+	 This in turn will cause the signal handling code in the
+	 kernel to recognize -ERESTARTSYS which will in turn cause the
+	 syscall to be restarted.
+
+	 The test case gdb.base/interrupt.exp tests for this problem.
+	 Without this sign extension code in place, it'll show
+	 a number of failures when testing against unix/-m32.  */
+
+      if (regnum == -1 || regnum == I386_EAX_REGNUM)
+	{
+	  void *ptr = ((gdb_byte *) gregs 
+		       + amd64_linux_gregset32_reg_offset[I386_EAX_REGNUM]);
+
+	  *(int64_t *) ptr = *(int32_t *) ptr;
+	}
+    }
+}
+
 /* Fill GDB's register cache with the general-purpose register values
    in *GREGSETP.  */
 
@@ -109,7 +174,7 @@ void
 fill_gregset (const struct regcache *regcache,
 	      elf_gregset_t *gregsetp, int regnum)
 {
-  amd64_collect_native_gregset (regcache, gregsetp, regnum);
+  amd64_linux_collect_native_gregset (regcache, gregsetp, regnum);
 }
 
 /* Transfering floating-point registers between GDB, inferiors and cores.  */
@@ -237,7 +302,7 @@ amd64_linux_nat_target::store_registers (struct regcache *regcache, int regnum)
       if (ptrace (PTRACE_GETREGS, tid, 0, (long) &regs) < 0)
 	perror_with_name (_("Couldn't get registers"));
 
-      amd64_collect_native_gregset (regcache, &regs, regnum);
+      amd64_linux_collect_native_gregset (regcache, &regs, regnum);
 
       if (ptrace (PTRACE_SETREGS, tid, 0, (long) &regs) < 0)
 	perror_with_name (_("Couldn't write registers"));