x86/fsgsbase/64: Introduce FS/GS base helper functions

Introduce FS/GS base access functionality via <asm/fsgsbase.h>,
not yet used by anything directly.

Factor out task_seg_base() from x86/ptrace.c and rename it to
x86_fsgsbase_read_task() to make it part of the new helpers.

This will allow us to enhance FSGSBASE support and eventually enable
the FSBASE/GSBASE instructions.

An "inactive" GS base refers to a base saved at kernel entry
and being part of an inactive, non-running/stopped user-task.
(The typical ptrace model.)

Here are the new functions:

  x86_fsbase_read_task()
  x86_gsbase_read_task()
  x86_fsbase_write_task()
  x86_gsbase_write_task()
  x86_fsbase_read_cpu()
  x86_fsbase_write_cpu()
  x86_gsbase_read_cpu_inactive()
  x86_gsbase_write_cpu_inactive()

As an advantage of the unified namespace we can now see all FS/GSBASE
API use in the kernel via the following 'git grep' pattern:

  $ git grep x86_.*sbase

[ mingo: Wrote new changelog. ]

Based-on-code-from: Andy Lutomirski <luto@kernel.org>
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1537312139-5580-3-git-send-email-chang.seok.bae@intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 5 insertions, 46 deletions

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 3acbf45cb7fb..fbde2a7ce377 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -39,7 +39,7 @@
 #include <asm/hw_breakpoint.h>
 #include <asm/traps.h>
 #include <asm/syscall.h>
-#include <asm/mmu_context.h>
+#include <asm/fsgsbase.h>
 
 #include "tls.h"
 
@@ -343,49 +343,6 @@ static int set_segment_reg(struct task_struct *task,
 	return 0;
 }
 
-static unsigned long task_seg_base(struct task_struct *task,
-				   unsigned short selector)
-{
-	unsigned short idx = selector >> 3;
-	unsigned long base;
-
-	if (likely((selector & SEGMENT_TI_MASK) == 0)) {
-		if (unlikely(idx >= GDT_ENTRIES))
-			return 0;
-
-		/*
-		 * There are no user segments in the GDT with nonzero bases
-		 * other than the TLS segments.
-		 */
-		if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
-			return 0;
-
-		idx -= GDT_ENTRY_TLS_MIN;
-		base = get_desc_base(&task->thread.tls_array[idx]);
-	} else {
-#ifdef CONFIG_MODIFY_LDT_SYSCALL
-		struct ldt_struct *ldt;
-
-		/*
-		 * If performance here mattered, we could protect the LDT
-		 * with RCU.  This is a slow path, though, so we can just
-		 * take the mutex.
-		 */
-		mutex_lock(&task->mm->context.lock);
-		ldt = task->mm->context.ldt;
-		if (unlikely(idx >= ldt->nr_entries))
-			base = 0;
-		else
-			base = get_desc_base(ldt->entries + idx);
-		mutex_unlock(&task->mm->context.lock);
-#else
-		base = 0;
-#endif
-	}
-
-	return base;
-}
-
 #endif	/* CONFIG_X86_32 */
 
 static unsigned long get_flags(struct task_struct *task)
@@ -482,13 +439,15 @@ static unsigned long getreg(struct task_struct *task, unsigned long offset)
 		if (task->thread.fsindex == 0)
 			return task->thread.fsbase;
 		else
-			return task_seg_base(task, task->thread.fsindex);
+			return x86_fsgsbase_read_task(task,
+						      task->thread.fsindex);
 	}
 	case offsetof(struct user_regs_struct, gs_base): {
 		if (task->thread.gsindex == 0)
 			return task->thread.gsbase;
 		else
-			return task_seg_base(task, task->thread.gsindex);
+			return x86_fsgsbase_read_task(task,
+						      task->thread.gsindex);
 	}
 #endif
 	}