x86/ptrace: Document FSBASE and GSBASE ABI oddities

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 3b3b169..f0e1ddb 100644 (file)
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -281,6 +281,20 @@ static int set_segment_reg(struct task_struct *task,
        if (invalid_selector(value))
                return -EIO;
 
+       /*
+        * This function has some ABI oddities.
+        *
+        * A 32-bit ptracer probably expects that writing FS or GS will change
+        * FSBASE or GSBASE respectively.  In the absence of FSGSBASE support,
+        * this code indeed has that effect.  When FSGSBASE is added, this
+        * will require a special case.
+        *
+        * For existing 64-bit ptracers, writing FS or GS *also* currently
+        * changes the base if the selector is nonzero the next time the task
+        * is run.  This behavior may not be needed, and trying to preserve it
+        * when FSGSBASE is added would be complicated at best.
+        */
+
        switch (offset) {
        case offsetof(struct user_regs_struct,fs):
                task->thread.fsindex = value;
@@ -370,6 +384,9 @@ static int putreg(struct task_struct *child,
                 * When changing the FS base, use do_arch_prctl_64()
                 * to set the index to zero and to set the base
                 * as requested.
+                *
+                * NB: This behavior is nonsensical and likely needs to
+                * change when FSGSBASE support is added.
                 */
                if (child->thread.fsbase != value)
                        return do_arch_prctl_64(child, ARCH_SET_FS, value);