From d733d1903370e746e7d4d7b82fed97894ba6ff08 Mon Sep 17 00:00:00 2001 From: Andy Lutomirski Date: Mon, 22 Mar 2021 10:26:47 +0800 Subject: [PATCH 2628/2944] x86/ptrace: Document FSBASE and GSBASE ABI oddities to #30648216 commit 56f2ab41b652251f336a0f471b1033afeaedd161 upstream. Signed-off-by: Andy Lutomirski Cc: Borislav Petkov Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Linus Torvalds Signed-off-by: Ingo Molnar Signed-off-by: Ethan Zhao Signed-off-by: Zelin Deng Reviewed-by: Artie Ding --- arch/x86/kernel/ptrace.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c index 975bdb5..d62f78c 100644 --- a/arch/x86/kernel/ptrace.c +++ b/arch/x86/kernel/ptrace.c @@ -304,6 +304,20 @@ 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; @@ -401,6 +415,9 @@ 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); -- 1.8.3.1