----- On Sep 19, 2018, at 1:10 PM, Joseph Myers joseph@xxxxxxxxxxxxxxxx wrote: > On Wed, 19 Sep 2018, Mathieu Desnoyers wrote: > >> > This looks like it's coming from the Linux kernel. Can't the relevant >> > uapi header just be used directly without copying into glibc (with due >> > care to ensure that glibc still builds if the kernel headers used for the >> > build are too old - you need such conditionals anyway if they don't define >> > the relevant syscall number)? >> >> This is indeed in the list of "things to consider" I've put in the patch >> commit message. If the usual practice is to build against uapi kernel headers >> outside of the glibc tree, I'm fine with that. > > We build with, currently, 3.2 or later headers (since 3.2 is EOL there's a > case for updating the minimum in glibc for both compile time and runtime, > but I haven't proposed that since there isn't much cleanup that would > enable and there's the open question of Carlos's proposal to eliminate the > runtime check on the kernel version and just let things try to run anyway > even if it's older than the configured minimum). Are you saying glibc has an explicit check for the kernel version visible from /proc before using specific features ? If so, how can this work with the variety of feature backports we find in the distribution kernels out there ? Checking whether specific system calls return ENOSYS errors seems more flexible. > Functions depending on > new syscalls may return ENOSYS errors if the headers used to build glibc > were too old. Since this patch is providing a data interface rather than > functions that can set errno to ENOSYS, presumably you have some other way > of signalling unavailability which would apply both with a too-old kernel > at runtime and too-old headers at compile time. For too-old kernel at runtime, having rseq registration return ENOSYS leaves the the content of __rseq_abi->cpu_id at its initial value (RSEQ_CPU_ID_UNINITIALIZED = -1). For too-old headers at compile time, one possibility is that we don't event expose the __rseq_abi TLS symbol. OTOH, if we need to keep exposing it anyway for ABI consistency purposes, then we'd leave its cpu_id field at the initial value (-1). But that would require that we copy linux/rseq.h into the glibc source tree. Thoughts ? Thanks, Mathieu > > -- > Joseph S. Myers > joseph@xxxxxxxxxxxxxxxx -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
