Messages in this thread Patch in this message | | | Date | Wed, 27 Nov 2019 08:19:28 -0000 | From | "tip-bot2 for Andy Lutomirski" <> | Subject | [tip: x86/urgent] x86/ptrace: Document FSBASE and GSBASE ABI oddities |
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The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: 56f2ab41b652251f336a0f471b1033afeaedd161 Gitweb: https://git.kernel.org/tip/56f2ab41b652251f336a0f471b1033afeaedd161 Author: Andy Lutomirski <luto@kernel.org> AuthorDate: Wed, 17 Jul 2019 06:44:16 -07:00 Committer: Ingo Molnar <mingo@kernel.org> CommitterDate: Tue, 26 Nov 2019 22:00:12 +01:00
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> --- 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 3b3b169..f0e1ddb 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);
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