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Message-ID: <CALCETrU5O66hT0fwRLP-hwJ9_CK1Q4F2vSXRXZPQ1Je7Y1CKoQ@mail.gmail.com>
Date: Mon, 25 Apr 2016 09:53:43 -0700
From: Andy Lutomirski <luto@...capital.net>
To: Dmitry Safonov <dsafonov@...tuozzo.com>,
Oleg Nesterov <oleg@...hat.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Dmitry Safonov <0x7f454c46@...il.com>,
Andy Lutomirski <luto@...nel.org>,
Ingo Molnar <mingo@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>
Subject: Re: [RFC 3/3] x86/ptrace: down with test_thread_flag(TIF_IA32)
On Mon, Apr 25, 2016 at 9:12 AM, Dmitry Safonov <dsafonov@...tuozzo.com> wrote:
> As the task isn't executing at the moment of {GET,SET}REGS,
> return regset that corresponds to code selector.
> So, for i386 elf binary that changed it's CS to __USER_CS
> it will return full x86_64 register set.
>
> That will change ABI: i.e, strace uses returned register size
> to determine, in which mode the application is.
> With the current ABI that way is buggy:
Oleg, any comment here?
>
> int main(int argc, char **argv, char **envp)
> {
> printf("Here we exit\n");
> fflush(stdout);
> asm volatile ("int $0x80" : : "a" (__NR_exit), "D" (1));
> printf("After exit\n");
>
> return 0;
> }
>
> This program will confuse strace:
>
> [tst]$ strace ./confuse 2>&1 | tail
> brk(0x1ca1000) = 0x1ca1000
> write(1, "Here we exit\n", 13Here we exit
> ) = 13
> exit(1) = ?
> <... exit resumed> strace: _exit returned!
> ) = ?
> write(1, "After exit\n", 11After exit
> ) = 11
> exit_group(0) = ?
> +++ exited with 0 +++
>
> So this ABI change should make PTRACE_GETREGSET more reliable and
> this will be another step to drop TIF_{IA32,X32} flags.
Does strace start working again with this change? I suspect that
we'll eventually have to expose syscall_get_arch directly through
ptrace, but that's a project for another day.
I think this patch is fine, but I'm not a ptrace expert.
--Andy
>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: x86@...nel.org
> Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
> ---
> arch/x86/kernel/ptrace.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index 0f4d2a5df2dc..d7d72f2f8b46 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -1387,7 +1387,7 @@ void update_regset_xstate_info(unsigned int size, u64 xstate_mask)
> const struct user_regset_view *task_user_regset_view(struct task_struct *task)
> {
> #ifdef CONFIG_IA32_EMULATION
> - if (test_tsk_thread_flag(task, TIF_IA32))
> + if (!user_64bit_mode(task_pt_regs(task)))
> #endif
> #if defined CONFIG_X86_32 || defined CONFIG_IA32_EMULATION
> return &user_x86_32_view;
> --
> 2.8.0
>
--
Andy Lutomirski
AMA Capital Management, LLC
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