* Mathieu Desnoyers: > The larger question here is: considering that we re-implement the entire > uapi header within glibc (which includes the uptr addition), do we still > care about using the header provided by the Linux kernel ? We don't care, but our users do. Eventually, they want to include <sys/rseq.h> and <linux/rseq.h> to get new constants that are not yet known to glibc. > Having different definitions depending on whether a kernel header is > installed or not when including a glibc header seems rather unexpected. Indeed. > *If* we want to use the uapi header, I think something is semantically > missing. Here is the scheme I envision. We could rely on the kernel header > version.h to figure out which of glibc or kernel uapi header is more > recent. Any new concept we try to integrate into glibc (e.g. uptr) > should go into the upstream Linux uapi header first. I think we should always prefer the uapi header. The Linux version check does not tell you anything about backports. > For the coming glibc e.g. 2.32, we use the kernel uapi header if > kernel version is >= 4.18.0. Within glibc, the fallback implements > exactly the API exposed by the kernel rseq.h header. Agreed. > As we eventually introduce the uptr change into the Linux kernel, and > say it gets merged for Linux 5.9.0, we mirror this change into glibc > (e.g. release 2.33), and bump the Linux kernel version cutoff to 5.9.0. > So starting from that version, we use the Linux kernel header only if > version >= 5.9.0, else we fallback on glibc's own implementation. Fortunately, we don't need to settle this today. 8-) Let's stick to the 4.18 definitions for the fallback for now, and discuss the incorporation of future changes later. >>> +/* Ensure the compiler supports __attribute__ ((aligned)). */ >>> +_Static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); >>> +_Static_assert (__alignof__ (struct rseq) >= 32, "alignment"); >> >> This needs #ifndef __cplusplus or something like that. I'm surprised >> that this passes the installed header tests. > > Would the following be ok ? > > #ifdef __cplusplus > #define rseq_static_assert static_assert > #else > #define rseq_static_assert _Static_assert > #endif > > /* Ensure the compiler supports __attribute__ ((aligned)). */ > rseq_static_assert (__alignof__ (struct rseq_cs) >= 32, "alignment"); > rseq_static_assert (__alignof__ (struct rseq) >= 32, "alignment"); Seems reasonable, yes. __alignof__ is still a GCC extension. C++11 has alignof, C11 has _Alignof. So you could use something like this (perhaps without indentation for the kernel header version): #ifdef __cplusplus # if __cplusplus >= 201103L # define rseq_static_assert(x) static_assert x; # define rseq_alignof alignof # endif #elif __STDC_VERSION__ >= 201112L # define rseq_static_assert(x) _Static_assert x; # define rseq_alignof _Alignof #endif #ifndef rseq_static_assert # define rseq_static_assert /* nothing */ #endif rseq_static_assert ((rseq_alignof__ (struct rseq_cs) >= 32, "alignment")) rseq_static_assert ((rseq_alignof (struct rseq) >= 32, "alignment")) And something similar for _Alignas/attribute aligned, with an error for older standards and !__GNUC__ compilers (because neither the type nor __thread can be represented there). Thanks, Florian
