diff options
| author | Andy Lutomirski | 2019-07-17 06:44:16 -0700 |
|---|---|---|
| committer | Ingo Molnar | 2019-11-26 22:00:12 +0100 |
| commit | 56f2ab41b652251f336a0f471b1033afeaedd161 (patch) | |
| tree | e9bd465d32e24be1a97e282ed3df6e25442681ac /arch/x86 | |
| parent | 8e05f1b4f27d07a0f93e7c6fd28525a5d082b85c (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.c | 17 |
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); |