x86/ptrace: Document FSBASE and GSBASE ABI oddities (56f2ab41) · Commits · 戴 / test

arch/x86/kernel/ptrace.c

+17 −0

Original line number	Diff line number	Diff line
		@@ -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);