Re: [PATCH] LoongArch: Store syscall nr in thread_info

[Hengqi Chen <hengqi.chen@gmail.com>]

On Wed, Nov 22, 2023 at 3:58 PM Huacai Chen <chenhuacai@kernel.org> wrote:
>
> Hi, Hengqi,
>
> On Wed, Nov 22, 2023 at 3:34 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
> >
> > Hi, Huacai,
> >
> > On Wed, Nov 22, 2023 at 2:32 PM Huacai Chen <chenhuacai@kernel.org> wrote:
> > >
> > > Hi, Hengqi,
> > >
> > > On Wed, Nov 22, 2023 at 1:14 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
> > > >
> > > > Currently, we store syscall number in pt_regs::regs[11] and it may be
> > > > changed during syscall execution. Take `execve` as an example:
> > > >
> > > >     sys_execve
> > > >       -> do_execve
> > > >         -> do_execveat_common
> > > >           -> bprm_execve
> > > >             -> exec_binprm
> > > >               -> search_binary_handler
> > > >                 -> load_elf_binary
> > > >                   -> ELF_PLAT_INIT
> > > >
> > > > ELF_PLAT_INIT reset regs[11] to 0, later in syscall_exit_to_user_mode
> > > > we get a wrong syscall nr.
> > > >
> > > > Known affected syscalls includes execve/execveat/rt_sigreturn. Tools
> > > > like execsnoop do not work properly because the sys_exit_* tracepoints
> > > > does not trigger at all.
> > > >
> > > > Let's store syscall nr in thread_info instead.
> > > Can we just modify ELF_PLAT_INIT and not clear regs[11]?
> > >
> >
> > I am uncertain about the side effects of changing ELF_PLAT_INIT.
> > From a completeness perspective, changing ELF_PLAT_INIT is suboptimal,
> > rt_sigreturn is affected in another code path, and there may be other
> > syscalls that I am unaware of.
> Save syscall number in thread_info has more side effects, because
> ptrace allows us to change the number during syscall, then we should
> keep consistency between syscall and regs[11].
>

How about the change below:

diff --git a/arch/loongarch/include/asm/syscall.h
b/arch/loongarch/include/asm/syscall.h
index e286dc58476e..954ba53bcc9a 100644
--- a/arch/loongarch/include/asm/syscall.h
+++ b/arch/loongarch/include/asm/syscall.h
@@ -23,7 +23,9 @@ extern void *sys_call_table[];
 static inline long syscall_get_nr(struct task_struct *task,
                                  struct pt_regs *regs)
 {
-       return regs->regs[11];
+       long nr = task_thread_info(task)->syscall;
+
+       return nr ? : regs->regs[11];
 }

 static inline void syscall_rollback(struct task_struct *task,
diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
index b4c5acd7aa3b..553ab0d624cb 100644
--- a/arch/loongarch/kernel/syscall.c
+++ b/arch/loongarch/kernel/syscall.c
@@ -53,6 +53,7 @@ void noinstr do_syscall(struct pt_regs *regs)
        regs->regs[4] = -ENOSYS;

        nr = syscall_enter_from_user_mode(regs, nr);
+       current_thread_info()->syscall = nr;

        if (nr < NR_syscalls) {
                syscall_fn = sys_call_table[nr];
@@ -61,4 +62,5 @@ void noinstr do_syscall(struct pt_regs *regs)
        }

        syscall_exit_to_user_mode(regs);
+       current_thread_info()->syscall = 0;
 }

* allow ptrace to change syscall nr
* sys_exit_* will also see the right syscall nr
* this works even if rt_sigreturn clobbers all pt_regs::regs

> And about ELF_PLAT_INIT, maybe Arnd can give us some more information.
>
> Hi, Arnd,
>
> I found some new architectures, such as ARM64 and RISC-V, just do
> nearly nothing in ELF_PLAT_INIT, while some old architectures, such as
> x86 and MIPS, clear most of the registers, do you know why?
>
> Huacai
>
> >
> > > Huacai
> > >
> > > >
> > > > Fixes: be769645a2aef ("LoongArch: Add system call support")
> > > > Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
> > > > ---
> > > >  arch/loongarch/include/asm/syscall.h | 2 +-
> > > >  arch/loongarch/kernel/syscall.c      | 1 +
> > > >  2 files changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/arch/loongarch/include/asm/syscall.h b/arch/loongarch/include/asm/syscall.h
> > > > index e286dc58476e..2317d674b92a 100644
> > > > --- a/arch/loongarch/include/asm/syscall.h
> > > > +++ b/arch/loongarch/include/asm/syscall.h
> > > > @@ -23,7 +23,7 @@ extern void *sys_call_table[];
> > > >  static inline long syscall_get_nr(struct task_struct *task,
> > > >                                   struct pt_regs *regs)
> > > >  {
> > > > -       return regs->regs[11];
> > > > +       return task_thread_info(task)->syscall;
> > > >  }
> > > >
> > > >  static inline void syscall_rollback(struct task_struct *task,
> > > > diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/syscall.c
> > > > index b4c5acd7aa3b..2783e33cf276 100644
> > > > --- a/arch/loongarch/kernel/syscall.c
> > > > +++ b/arch/loongarch/kernel/syscall.c
> > > > @@ -52,6 +52,7 @@ void noinstr do_syscall(struct pt_regs *regs)
> > > >         regs->orig_a0 = regs->regs[4];
> > > >         regs->regs[4] = -ENOSYS;
> > > >
> > > > +       task_thread_info(current)->syscall = nr;
> > > >         nr = syscall_enter_from_user_mode(regs, nr);
> > > >
> > > >         if (nr < NR_syscalls) {
> > > > --
> > > > 2.42.0
> > > >