Subject: Re: [PATCH v10 5/7] signal: deduplicate code dealing with common _sigfault fields

From: Dave Martin <[email-masked]>
Date: Wed, 7 Oct 2020 09:56:06 +0100

On Mon, Oct 05, 2020 at 10:07:01PM -0700, Peter Collingbourne wrote:
> On Tue, Sep 8, 2020 at 8:13 AM Dave Martin <[email-masked]> wrote:
> >
> > On Fri, Aug 21, 2020 at 10:10:15PM -0700, Peter Collingbourne wrote:
> > > We're about to add more common _sigfault fields, so deduplicate the
> > > existing code for initializing _sigfault fields in {send,force}_sig_*,
> > > and for copying _sigfault fields in copy_siginfo_to_external32 and
> > > post_copy_siginfo_from_user32, to reduce the number of places that
> > > will need to be updated by upcoming changes.
> > >
> > > Signed-off-by: Peter Collingbourne <[email-masked]>
> > > ---
> > > View this change in Gerrit: https://linux-review.googlesource.com/q/I4f56174e1b7b2bf4a3c8139e6879cbfd52750a24
> > >
> > >  include/linux/signal.h |  13 ++++++
> > >  kernel/signal.c        | 101 ++++++++++++++++-------------------------
> > >  2 files changed, 53 insertions(+), 61 deletions(-)
> > >
> > > diff --git a/include/linux/signal.h b/include/linux/signal.h
> > > index 6bb1a3f0258c..3edbf54493ee 100644
> > > --- a/include/linux/signal.h
> > > +++ b/include/linux/signal.h
> > > @@ -50,6 +50,19 @@ enum siginfo_layout {
> > >
> > >  enum siginfo_layout siginfo_layout(unsigned sig, int si_code);
> > >
> > > +static inline bool siginfo_layout_is_fault(enum siginfo_layout layout)
> > > +{
> > > +     switch (layout) {
> > > +     case SIL_FAULT:
> > > +     case SIL_FAULT_MCEERR:
> > > +     case SIL_FAULT_BNDERR:
> > > +     case SIL_FAULT_PKUERR:
> > > +             return true;
> > > +     default:
> > > +             return false;
> > > +     }
> > > +}
> > > +
> > >  /*
> > >   * Define some primitives to manipulate sigset_t.
> > >   */
> > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > index c80e70bde11d..4ee9dc03f20f 100644
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -1649,6 +1649,15 @@ void force_sigsegv(int sig)
> > >       force_sig(SIGSEGV);
> > >  }
> > >
> > > +static void set_sigfault_common_fields(struct kernel_siginfo *info, int sig,
> > > +                                    int code, void __user *addr)
> > > +{
> > > +     info->si_signo = sig;
> > > +     info->si_errno = 0;
> > > +     info->si_code = code;
> > > +     info->si_addr = addr;
> > > +}
> > > +
> > >  int force_sig_fault_to_task(int sig, int code, void __user *addr
> > >       ___ARCH_SI_TRAPNO(int trapno)
> > >       ___ARCH_SI_IA64(int imm, unsigned int flags, unsigned long isr)
> > > @@ -1657,10 +1666,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
> > >       struct kernel_siginfo info;
> > >
> > >       clear_siginfo(&info);
> > > -     info.si_signo = sig;
> > > -     info.si_errno = 0;
> > > -     info.si_code  = code;
> > > -     info.si_addr  = addr;
> > > +     set_sigfault_common_fields(&info, sig, code, addr);
> > >  #ifdef __ARCH_SI_TRAPNO
> > >       info.si_trapno = trapno;
> > >  #endif
> > > @@ -1689,10 +1695,7 @@ int send_sig_fault(int sig, int code, void __user *addr
> > >       struct kernel_siginfo info;
> > >
> > >       clear_siginfo(&info);
> > > -     info.si_signo = sig;
> > > -     info.si_errno = 0;
> > > -     info.si_code  = code;
> > > -     info.si_addr  = addr;
> > > +     set_sigfault_common_fields(&info, sig, code, addr);
> > >  #ifdef __ARCH_SI_TRAPNO
> > >       info.si_trapno = trapno;
> > >  #endif
> > > @@ -1710,10 +1713,7 @@ int force_sig_mceerr(int code, void __user *addr, short lsb)
> > >
> > >       WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
> > >       clear_siginfo(&info);
> > > -     info.si_signo = SIGBUS;
> > > -     info.si_errno = 0;
> > > -     info.si_code = code;
> > > -     info.si_addr = addr;
> > > +     set_sigfault_common_fields(&info, SIGBUS, code, addr);
> > >       info.si_addr_lsb = lsb;
> > >       return force_sig_info(&info);
> > >  }
> > > @@ -1724,10 +1724,7 @@ int send_sig_mceerr(int code, void __user *addr, short lsb, struct task_struct *
> > >
> > >       WARN_ON((code != BUS_MCEERR_AO) && (code != BUS_MCEERR_AR));
> > >       clear_siginfo(&info);
> > > -     info.si_signo = SIGBUS;
> > > -     info.si_errno = 0;
> > > -     info.si_code = code;
> > > -     info.si_addr = addr;
> > > +     set_sigfault_common_fields(&info, SIGBUS, code, addr);
> > >       info.si_addr_lsb = lsb;
> > >       return send_sig_info(info.si_signo, &info, t);
> > >  }
> > > @@ -1738,10 +1735,7 @@ int force_sig_bnderr(void __user *addr, void __user *lower, void __user *upper)
> > >       struct kernel_siginfo info;
> > >
> > >       clear_siginfo(&info);
> > > -     info.si_signo = SIGSEGV;
> > > -     info.si_errno = 0;
> > > -     info.si_code  = SEGV_BNDERR;
> > > -     info.si_addr  = addr;
> > > +     set_sigfault_common_fields(&info, SIGSEGV, SEGV_BNDERR, addr);
> > >       info.si_lower = lower;
> > >       info.si_upper = upper;
> > >       return force_sig_info(&info);
> > > @@ -1753,10 +1747,7 @@ int force_sig_pkuerr(void __user *addr, u32 pkey)
> > >       struct kernel_siginfo info;
> > >
> > >       clear_siginfo(&info);
> > > -     info.si_signo = SIGSEGV;
> > > -     info.si_errno = 0;
> > > -     info.si_code  = SEGV_PKUERR;
> > > -     info.si_addr  = addr;
> > > +     set_sigfault_common_fields(&info, SIGSEGV, SEGV_PKUERR, addr);
> > >       info.si_pkey  = pkey;
> > >       return force_sig_info(&info);
> > >  }
> > > @@ -1770,10 +1761,8 @@ int force_sig_ptrace_errno_trap(int errno, void __user *addr)
> > >       struct kernel_siginfo info;
> > >
> > >       clear_siginfo(&info);
> > > -     info.si_signo = SIGTRAP;
> > > +     set_sigfault_common_fields(&info, SIGTRAP, TRAP_HWBKPT, addr);
> > >       info.si_errno = errno;
> > > -     info.si_code  = TRAP_HWBKPT;
> > > -     info.si_addr  = addr;
> > >       return force_sig_info(&info);
> > >  }
> > >
> > > @@ -3266,12 +3255,23 @@ int copy_siginfo_from_user(kernel_siginfo_t *to, const siginfo_t __user *from)
> > >  void copy_siginfo_to_external32(struct compat_siginfo *to,
> > >               const struct kernel_siginfo *from)
> > >  {
> > > +     enum siginfo_layout layout =
> > > +             siginfo_layout(from->si_signo, from->si_code);
> > > +
> > >       memset(to, 0, sizeof(*to));
> > >
> > >       to->si_signo = from->si_signo;
> > >       to->si_errno = from->si_errno;
> > >       to->si_code  = from->si_code;
> > > -     switch(siginfo_layout(from->si_signo, from->si_code)) {
> > > +
> > > +     if (siginfo_layout_is_fault(layout)) {
> > > +             to->si_addr = ptr_to_compat(from->si_addr);
> > > +#ifdef __ARCH_SI_TRAPNO
> > > +             to->si_trapno = from->si_trapno;
> > > +#endif
> > > +     }
> > > +
> > > +     switch (layout) {
> >
> > I find the code flow slightly awkward with this change, because the
> > fault signal fields are populated partly in the if() above and partly
> > in the switch().  Previously, only the universal stuff was done outside.
> >
> > Would this be easier on future maintainers if we pulled the common
> > stuff out into a helper and then called it from the appropriate switch
> > cases?  The compiler will probably output some duplicated code in that
> > case (depending on how clever it is at undoing the duplication), but
> > the amount of affected code is small.
> 
> I'm not sure about that. One advantage of the current code structure
> is that we end up with siginfo_layout_is_fault containing our single
> canonical list of layouts that use the sigfault union member. With
> your proposed code structure, the only caller of
> siginfo_layout_is_fault would be the code that the next patch adds to
> kernel/ptrace.c, which needs to know which layouts use sigfault so
> that it can clear si_xflags, and that could relatively easily get out
> of sync by accident since it's dealing with a less common case.
> 
> That being said, perhaps we could say that a caller of
> ptrace(PTRACE_SETSIGINFO) is by definition old, so it wouldn't be
> using that API to send signals with new siginfo layouts. That would
> preclude a client from upgrading its knowledge of si_xflags
> independently of its knowledge of new layouts though, and there would
> be nothing preventing a new caller of siginfo_layout_is_fault being
> added that would be exposed to new user code.

OK, I guess I don't have a strong view on this for now.

I suppose that clear_siginfo() could be moved into
set_sigfault_common_fields() (perhaps warranting a rename to
init_sigfault() or similar).

But that's just one line per case, so probably not worth getting excited
about.  There is also some virtue in keeping the clear_siginfo()
explicit everywhere, so that people are reminded about the need for it.

Anyway, that's not essential.

[...]

Cheers
---Dave

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