Re: [PATCH 10/12] signal: Redefine signinfo so 64bit fields are possible

[Marco Elver <elver@google.com>]

On Mon, 3 May 2021 at 23:04, Eric W. Biederman <ebiederm@xmission.com> wrote:
> "Eric W. Beiderman" <ebiederm@xmission.com> writes:
> > From: "Eric W. Biederman" <ebiederm@xmission.com>
> >
> > The si_perf code really wants to add a u64 field.  This change enables
> > that by reorganizing the definition of siginfo_t, so that a 64bit
> > field can be added without increasing the alignment of other fields.

If you can, it'd be good to have an explanation for this, because it's
not at all obvious -- some future archeologist will wonder how we ever
came up with this definition of siginfo...

(I see the trick here is that before the union would have changed
alignment, introducing padding after the 3 ints -- but now because the
3 ints are inside the union the union's padding no longer adds padding
for these ints.  Perhaps you can explain it better than I can. Also
see below.)

> I decided to include this change because it is not that complicated,
> and it allows si_perf_data to have the definition that was originally
> desired.

Neat, and long-term I think this seems to be worth having. Sooner or
later something else might want __u64, too.

But right now, due to the slight increase in complexity, we need to
take extra care ensuring nothing broke -- at a high-level I see why
this seems to be safe.

> If this looks difficult to make in the userspace definitions,
> or is otherwise a problem I don't mind dropping this change.  I just
> figured since it was not too difficult and we are changing things
> anyway I should try for everything.

Languages that support inheritance might end up with the simpler
definition here (and depending on which fields they want to access,
they'd have to cast the base siginfo into the one they want). What
will become annoying is trying to describe siginfo_t.

I will run some tests in the morning.

Thanks,
-- Marco

[...]
> > diff --git a/include/uapi/asm-generic/siginfo.h b/include/uapi/asm-generic/siginfo.h
> > index e663bf117b46..1fcede623a73 100644
> > --- a/include/uapi/asm-generic/siginfo.h
> > +++ b/include/uapi/asm-generic/siginfo.h
> > @@ -29,15 +29,33 @@ typedef union sigval {
> >  #define __ARCH_SI_ATTRIBUTES
> >  #endif
> >
> > +#ifndef __ARCH_HAS_SWAPPED_SIGINFO
> > +#define ___SIGINFO_COMMON    \
> > +     int     si_signo;       \
> > +     int     si_errno;       \
> > +     int     si_code
> > +#else
> > +#define ___SIGINFO_COMMON    \
> > +     int     si_signo;       \
> > +     int     si_code;        \
> > +     int     si_errno
> > +#endif /* __ARCH_HAS_SWAPPED_SIGINFO */
> > +
> > +#define __SIGINFO_COMMON     \
> > +     ___SIGINFO_COMMON;      \
> > +     int     _common_pad[__alignof__(void *) != __alignof(int)]

Just to verify my understanding of _common_pad: this is again a legacy
problem, right? I.e. if siginfo was designed from the start like this,
we wouldn't need the _common_pad.


While this looks quite daunting, this is effectively turning siginfo
from a struct with a giant union, into lots of smaller structs, each
of which share a common header a'la inheritance -- until now the
distinction didn't matter, but it starts to matter as soon as
alignment of one child-struct would affect the offsets of another
child-struct (i.e. the old version). Right?

I was wondering if it would make things look cleaner if the above was
encapsulated in a struct, say __sicommon? Then the outermost union
would use 'struct __sicommon _sicommon' and we need #defines for
si_signo, si_code, and si_errno.

Or would it break alignment somewhere?

I leave it to your judgement -- just an idea.

> >  union __sifields {
> >       /* kill() */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               __kernel_pid_t _pid;    /* sender's pid */
> >               __kernel_uid32_t _uid;  /* sender's uid */
> >       } _kill;
> >
> >       /* POSIX.1b timers */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               __kernel_timer_t _tid;  /* timer id */
> >               int _overrun;           /* overrun count */
> >               sigval_t _sigval;       /* same as below */
> > @@ -46,6 +64,7 @@ union __sifields {
> >
> >       /* POSIX.1b signals */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               __kernel_pid_t _pid;    /* sender's pid */
> >               __kernel_uid32_t _uid;  /* sender's uid */
> >               sigval_t _sigval;
> > @@ -53,6 +72,7 @@ union __sifields {
> >
> >       /* SIGCHLD */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               __kernel_pid_t _pid;    /* which child */
> >               __kernel_uid32_t _uid;  /* sender's uid */
> >               int _status;            /* exit code */
> > @@ -62,6 +82,7 @@ union __sifields {
> >
> >       /* SIGILL, SIGFPE, SIGSEGV, SIGBUS, SIGTRAP, SIGEMT */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               void __user *_addr; /* faulting insn/memory ref. */
> >  #ifdef __ia64__
> >               int _imm;               /* immediate value for "break" */
> > @@ -97,35 +118,28 @@ union __sifields {
> >
> >       /* SIGPOLL */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               __ARCH_SI_BAND_T _band; /* POLL_IN, POLL_OUT, POLL_MSG */
> >               int _fd;
> >       } _sigpoll;
> >
> >       /* SIGSYS */
> >       struct {
> > +             __SIGINFO_COMMON;
> >               void __user *_call_addr; /* calling user insn */
> >               int _syscall;   /* triggering system call number */
> >               unsigned int _arch;     /* AUDIT_ARCH_* of syscall */
> >       } _sigsys;
> >  };
> >
> > -#ifndef __ARCH_HAS_SWAPPED_SIGINFO
> > -#define __SIGINFO                    \
> > -struct {                             \
> > -     int si_signo;                   \
> > -     int si_errno;                   \
> > -     int si_code;                    \
> > -     union __sifields _sifields;     \
> > -}
> > -#else
> > +
> >  #define __SIGINFO                    \
> > -struct {                             \
> > -     int si_signo;                   \
> > -     int si_code;                    \
> > -     int si_errno;                   \
> > +union {                                      \
> > +     struct {                        \
> > +             __SIGINFO_COMMON;       \
> > +     };                              \
> >       union __sifields _sifields;     \
> >  }
> > -#endif /* __ARCH_HAS_SWAPPED_SIGINFO */
> >
> >  typedef struct siginfo {
> >       union {