Date: Tue, 8 Dec 2015 21:13:24 +0200 From: Konstantin Belousov <kostikbel@gmail.com> To: Dag-Erling Sm??rgrav <des@des.no> Cc: Kirk McKusick <mckusick@mckusick.com>, Maxim Sobolev <sobomax@FreeBSD.org>, freebsd-current@freebsd.org, freebsd-hackers@freebsd.org Subject: Re: posix_fallocate(2) && posix_fadvise(2) are somewhat broken Message-ID: <20151208191324.GF82577@kib.kiev.ua> In-Reply-To: <86poygrctt.fsf@desk.des.no> References: <CAH7qZfvV-RepAc6N0UxFi2RBthxrd%2BqHD-Qh5dc-9v=NFGCy_w@mail.gmail.com> <868u55rl96.fsf@desk.des.no> <20151208174259.GA82577@kib.kiev.ua> <86poygrctt.fsf@desk.des.no>
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On Tue, Dec 08, 2015 at 07:54:06PM +0100, Dag-Erling Sm??rgrav wrote: > Konstantin Belousov <kostikbel@gmail.com> writes: > > Dag-Erling Sm??rgrav <des@des.no> writes: > > > Maxim Sobolev <sobomax@FreeBSD.org> writes: > > > > Hi, while working on some unrelated feature I've noticed that at least > > > > those two system calls are not returning proper value (-1) on error. > > > > Instead actual errno value is returned from the syscall verbatim, > > > > i.e. posix_fadvise() returns 22 on EINVAL. > > > That's how syscalls work. > > No, this is not how typical syscalls work, but is how the posix_fallocate() > > and posix_fadvise() are specified by Posix. The patch is wrong, see also > > r261080 and r288640. > > Umm, I can't find the code ATM but syscalls store the actual return > value in td_retval and return 0 or EWHATEVER and the syscall wrapper > handles the translation. If that's not what Maxim was talking about, > then please ignore me. I mean that typical syscall does not return error to usermode, it returns -1 and sets errno. But usermode conventions for the posix_f*e() are different, and I believe this is what tripped over Maxim and I reacted upon. Indeed kernel expects the syscall function from the sysentvec table to return error or zero. If zero is returned, then td_retval array is translated into return value for usermode by cpu_set_syscall_retval(). If non-zero is returned, typical kernel/libc interface returns the syscall function return value to usermode and additionally set flag (like PSL_C in the processor status word). Of course, there is an additional translation layer in usermode syscall trampolines. > > Anyway, happy to hear that the X/Open group have found a new way to > screw people over. It is the same as the pthread_* conventions. They are somewhat consistent.
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