Re: [PATCH glibc 1/3] glibc: Perform rseq registration at C startup and thread creation (v19)

----- On May 26, 2020, at 10:38 AM, Florian Weimer fweimer@redhat.com wrote:

* Mathieu Desnoyers:

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AFAIU, the only gain here would be to make sure we don't emit useless
";" in the "/* nothing */" case. But does it matter ?

I don't think C allows empty constructs like this at the top level.

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And something similar for _Alignas/attribute aligned,

I don't see where _Alignas is needed here ?

For attribute aligned, what would be the oldest supported C and C++
standards ?

There are no standardized attributes for C, there is only _Alignas.
C++11 has an alignas specifier; it's not an attribute either.  I think
these are syntactically similar.

There appears to be an interesting difference between attribute aligned
and alignas. It seems like alignas cannot be used on a structure declaration,
only on fields, e.g.:

struct blah {
        int a;
} _Alignas (16);

o.c:3:1: warning: useless ‘_Alignas’ in empty declaration
 } _Alignas (16);

But

struct blah {
        int _Alignas (16) a;
};

Like the attribute, it needs to come right after the struct keyword, I
think.  (Trailing attributes can be ambiguous, but not in this case.)

Nope. _Alignas really _is_ special :-(

struct _Alignas (16) blah {
        int a;
};

p.c:1:8: error: expected ‘{’ before ‘_Alignas’
 struct _Alignas (16) blah {

Also:

struct blah _Alignas (16) {
        int a;
};

p.c:1:27: error: expected identifier or ‘(’ before ‘{’ token
 struct blah _Alignas (16) {

quoted

is OK. So if I change e.g. struct rseq_cs to align
the first field:

struct rseq_cs
  {
    /* Version of this structure.  */
    uint32_t rseq_align (32) version;
    /* enum rseq_cs_flags.  */
    uint32_t flags;
    uint64_t start_ip;
    /* Offset from start_ip.  */
    uint64_t post_commit_offset;
    uint64_t abort_ip;
  };

It should work.

Indeed.

OK, so let's go for that approach.

quoted

/* Rely on GNU extensions for older standards and tls model.  */
#ifdef __GNUC__
# ifndef rseq_alignof
#  define rseq_alignof(x) __alignof__ (x)
# endif
# ifndef rseq_alignas
#  define rseq_alignas(x) __attribute__ ((aligned (x)))
# endif
# define rseq_tls_model_ie __attribute__ ((__tls_model__ ("initial-exec")))
#else
/* Specifying the TLS model on the declaration is optional.  */
# define rseq_tls_model_ie /* Nothing.  */
#endif

/* Fall back to __thread for TLS storage class.  */
#ifndef rseq_tls_storage_class
# define rseq_tls_storage_class __thread
#endif

If they are only used in the glibc headers, they should have __rseq
prefixes, so that application code doesn't start using them (in case we
have to change/fix them, or move the into <sys/cdefs.h> later).

OK will do.

Rest looks fine.

One last thing I'm planning to add in sys/rseq.h to cover acessing the
rseq_cs pointers with both the UAPI headers and the glibc struct rseq
declarations:

/* The rseq_cs_ptr macro can be used to access the pointer to the current
   rseq critical section descriptor.  */
#ifdef __LP64__
# define rseq_cs_ptr(rseq) \
           ((const struct rseq_cs *) (rseq)->rseq_cs.ptr)
#else /* __LP64__ */
# define rseq_cs_ptr(rseq) \
           ((const struct rseq_cs *) (rseq)->rseq_cs.ptr.ptr32)
#endif /* __LP64__ */

Does it make sense ?

Thanks,

Mathieu

Thanks,
Florian

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com