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[PATCH] Add support for "orig_eax" pseudo register on Linux/x86

This patch adds a new "pseudo" register to i?86-*-linux-gnu targets.
This isn't a real harware register, but it is exposed by the kernel.
I need to be able to diddle with it to fix a bug with running a
function from within GDB from an interrupted system call.

This patch doesn't add any code to actually set the "orig_eax"
register.  That'll come later.


Index: ChangeLog
from  Mark Kettenis  <>

	Add support for "orig_eax" pseudo register on Linux/x86.
	* i386-linux-tdep.c: Include "inferior.h".
	(i386_linux_register_name, i386_linux_register_byte,
	i386_linux_register_raw_size): New functions.
	(i386_linux_write_pc): New function.
	* config/i386/tm-linux.h (I386_LINUX_ORIG_EAX_REGNUM): New define.
	REGISTER_BYTE, REGISTER_RAW_SIZE): Define to deal with additional
	(i386_linux_register_name, i386_linux_register_byte,
	i386_linux_register_raw_size): New prototypes.
	(TARGET_WRITE_PC): New define.
	(i386_linux_write_pc): New prototype.

Index: i386-linux-tdep.c
RCS file: /cvs/src/src/gdb/i386-linux-tdep.c,v
retrieving revision 1.8
diff -u -p -r1.8 i386-linux-tdep.c
--- i386-linux-tdep.c 2001/04/17 09:38:52 1.8
+++ i386-linux-tdep.c 2001/11/04 13:58:25
@@ -23,6 +23,7 @@
 #include "frame.h"
 #include "value.h"
 #include "regcache.h"
+#include "inferior.h"
 /* For i386_linux_skip_solib_resolver.  */
 #include "symtab.h"
@@ -31,6 +32,38 @@
 #include "solib-svr4.h"		/* For struct link_map_offsets.  */
+/* Return the name of register REG.  */
+char *
+i386_linux_register_name (int reg)
+  /* Deal with the extra "orig_eax" pseudo register.  */
+  if (reg == I386_LINUX_ORIG_EAX_REGNUM)
+    return "orig_eax";
+  return i386_register_name (reg);
+i386_linux_register_byte (int reg)
+  /* Deal with the extra "orig_eax" pseudo register.  */
+  if (reg == I386_LINUX_ORIG_EAX_REGNUM)
+    return (i386_register_byte (I386_LINUX_ORIG_EAX_REGNUM - 1)
+	    + i386_register_raw_size (I386_LINUX_ORIG_EAX_REGNUM - 1));
+  return i386_register_byte (reg);
+i386_linux_register_raw_size (int reg)
+  /* Deal with the extra "orig_eax" pseudo register.  */
+  if (reg == I386_LINUX_ORIG_EAX_REGNUM)
+    return 4;
+  return i386_register_raw_size (reg);
 /* Recognizing signal handler frames.  */
@@ -342,9 +375,33 @@ i386_linux_saved_pc_after_call (struct f
   return read_memory_unsigned_integer (read_register (SP_REGNUM), 4);
+/* Set the program counter for process PTID to PC.  */
+i386_linux_write_pc (CORE_ADDR pc, ptid_t ptid)
+  write_register_pid (PC_REGNUM, pc, ptid);
+  /* We must be careful with modifying the program counter.  If we
+     just interrupted a system call, the kernel might try to restart
+     it when we resume the inferior.  On restarting the system call,
+     the kernel will try backing up the program counter even though it
+     no longer points at the system call.  This typically results in a
+     SIGSEGV or SIGILL.  We can prevent this by writing `-1' in the
+     "orig_eax" pseudo-register.
+     Note that "orig_eax" is saved when setting up a dummy call frame.
+     This means that it is properly restored when that frame is
+     popped, and that the interrupted system call will be restarted
+     when we resume the inferior on return from a function call from
+     within GDB.  In all other cases the system call will not be
+     restarted.  */
+  write_register_pid (I386_LINUX_ORIG_EAX_REGNUM, -1, ptid);
 /* Calling functions in shared libraries.  */
 /* Find the minimal symbol named NAME, and return both the minsym
    struct and its objfile.  This probably ought to be in minsym.c, but
    everything there is trying to deal with things like C++ and
Index: config/i386/tm-linux.h
RCS file: /cvs/src/src/gdb/config/i386/tm-linux.h,v
retrieving revision 1.14
diff -u -p -r1.14 tm-linux.h
--- config/i386/tm-linux.h 2001/07/30 10:17:46 1.14
+++ config/i386/tm-linux.h 2001/11/04 13:58:25
@@ -31,6 +31,35 @@
 #include "i386/tm-i386.h"
 #include "tm-linux.h"
+/* Register number for the "orig_eax" pseudo-register.  If this
+   pseudo-register contains a value >= 0 it is interpreted as the
+   system call number that the kernel is supposed to restart.  */
+#define I386_LINUX_ORIG_EAX_REGNUM 41
+/* Adjust a few macros to deal with this extra register.  */
+#undef NUM_REGS
+#undef MAX_NUM_REGS
+#define MAX_NUM_REGS (16 + 16 + 9 + 1)
+#define REGISTER_NAME(reg) i386_linux_register_name ((reg))
+extern char *i386_linux_register_name (int reg);
+#define REGISTER_BYTE(reg) i386_linux_register_byte ((reg))
+extern int i386_linux_register_byte (int reg);
+#define REGISTER_RAW_SIZE(reg) i386_linux_register_raw_size ((reg))
+extern int i386_linux_register_raw_size (int reg);
 /* Linux/ELF uses stabs-in-ELF with the DWARF register numbering
    scheme by default, so we must redefine STAB_REG_TO_REGNUM.  This
    messes up the floating-point registers for a.out, but there is not
@@ -64,6 +93,9 @@ extern CORE_ADDR i386_linux_frame_saved_
 #define SAVED_PC_AFTER_CALL(frame) i386_linux_saved_pc_after_call (frame)
 extern CORE_ADDR i386_linux_saved_pc_after_call (struct frame_info *);
+#define TARGET_WRITE_PC(pc, ptid) i386_linux_write_pc (pc, ptid)
+extern void i386_linux_write_pc (CORE_ADDR pc, ptid_t ptid);
 /* When we call a function in a shared library, and the PLT sends us
    into the dynamic linker to find the function's real address, we

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