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authorAndy Lutomirski <luto@kernel.org>2019-07-17 06:44:16 -0700
committerIngo Molnar <mingo@kernel.org>2019-11-26 22:00:12 +0100
commit56f2ab41b652251f336a0f471b1033afeaedd161 (patch)
treee9bd465d32e24be1a97e282ed3df6e25442681ac /arch/x86
parent8e05f1b4f27d07a0f93e7c6fd28525a5d082b85c (diff)
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>
Diffstat (limited to 'arch/x86')
-rw-r--r--arch/x86/kernel/ptrace.c17
1 files changed, 17 insertions, 0 deletions
diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 3b3b16932589..f0e1ddbc2fd7 100644
--- 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);