Date: Sun, 5 Dec 2021 17:14:54 +0000 From: Jessica Clarke <jrtc27@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: "src-committers@freebsd.org" <src-committers@freebsd.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@freebsd.org>, "dev-commits-src-main@freebsd.org" <dev-commits-src-main@freebsd.org> Subject: Re: git: a4e4132fa3bf - main - swapoff(2): replace special device name argument with a structure Message-ID: <CCBD810D-80DB-43ED-9957-4F9A9CB950E5@freebsd.org> In-Reply-To: <Yay8/x8lTm59vTlo@kib.kiev.ua> References: <202112042221.1B4ML7Ov002151@gitrepo.freebsd.org> <EE06FFF1-7587-4F6E-8649-63454155F2C8@freebsd.org> <Yay8/x8lTm59vTlo@kib.kiev.ua>
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On 5 Dec 2021, at 13:22, Konstantin Belousov <kostikbel@gmail.com> = wrote: >=20 > On Sun, Dec 05, 2021 at 03:03:26AM +0000, Jessica Clarke wrote: >> On 4 Dec 2021, at 22:21, Konstantin Belousov <kib@FreeBSD.org> wrote: >>>=20 >>> The branch main has been updated by kib: >>>=20 >>> URL: = https://cgit.FreeBSD.org/src/commit/?id=3Da4e4132fa3bfadb6047fc0fa5f399f46= 40460300 >>>=20 >>> commit a4e4132fa3bfadb6047fc0fa5f399f4640460300 >>> Author: Konstantin Belousov <kib@FreeBSD.org> >>> AuthorDate: 2021-11-29 16:26:31 +0000 >>> Commit: Konstantin Belousov <kib@FreeBSD.org> >>> CommitDate: 2021-12-04 22:20:58 +0000 >>>=20 >>> swapoff(2): replace special device name argument with a structure >>>=20 >>> For compatibility, add a placeholder pointer to the start of the >>> added struct swapoff_new_args, and use it to distinguish old vs. = new >>> style of syscall invocation. >>>=20 >>> Reviewed by: markj >>> Discussed with: alc >>> Sponsored by: The FreeBSD Foundation >>> MFC after: 1 week >>> Differential revision: https://reviews.freebsd.org/D33165 >>> --- >>> sys/vm/swap_pager.c | 27 +++++++++++++++++++++++++-- >>> sys/vm/swap_pager.h | 8 ++++++++ >>> 2 files changed, 33 insertions(+), 2 deletions(-) >>>=20 >>> diff --git a/sys/vm/swap_pager.c b/sys/vm/swap_pager.c >>> index 165373d1b527..dc1df79f4fcd 100644 >>> --- a/sys/vm/swap_pager.c >>> +++ b/sys/vm/swap_pager.c >>> @@ -2491,15 +2491,38 @@ sys_swapoff(struct thread *td, struct = swapoff_args *uap) >>> struct vnode *vp; >>> struct nameidata nd; >>> struct swdevt *sp; >>> - int error; >>> + struct swapoff_new_args sa; >>> + int error, probe_byte; >>>=20 >>> error =3D priv_check(td, PRIV_SWAPOFF); >>> if (error) >>> return (error); >>>=20 >>> + /* >>> + * Detect old vs. new-style swapoff(2) syscall. The first >>> + * pointer in the memory pointed to by uap->name is NULL for >>> + * the new variant. >>> + */ >>> + probe_byte =3D fubyte(uap->name); >>> + switch (probe_byte) { >>> + case -1: >>> + return (EFAULT); >>> + case 0: >>> + error =3D copyin(uap->name, &sa, sizeof(sa)); >>> + if (error !=3D 0) >>> + return (error); >>> + if (sa.flags !=3D 0) >>> + return (EINVAL); >>> + break; >>> + default: >>> + bzero(&sa, sizeof(sa)); >>> + sa.name =3D uap->name; >>> + break; >>> + } >>=20 >> Doesn=E2=80=99t this change the semantics of swapoff("")? >>=20 >> Previously it would fail deterministically, presumably with ENOENT or >> something, but now it reinterprets whatever follows that string in >> memory as the new argument structure. It probably doesn=E2=80=99t = matter, but >> this approach is ugly. Can we not just define a new syscall rather = than >> this kind of bodge? >=20 > Having two swapoff() syscalls is worse, and having them only differ in > semantic by single flag is kind of crime. >=20 > I do not see swapoff("") as problematic, we are changing a minor = semantic of > the management syscall. I only wanted to avoid flag day for swapoff = binaries. >=20 > BTW, I considered requiring proper alignment for uap->name, and then = checking > the whole uap->name_old_syscall for NULL, but then decided that this = is > overkill. If you think that swapoff("") that important, I can add = that > additional verification. Why=E2=80=99s it worse? It=E2=80=99s just a syscall number, you = deprecate the old one and move on, we do that for things relatively regularly. This is really not a good solution; harder to use as a caller since the prototype is wrong, impossible to ensure you preserve the semantics for the existing interface in all cases, and ugly to implement. You don=E2=80=99t need a = flag day for a new syscall, either, you can continue to only use the new method for -f for a release and then switch over to the new syscall entirely. Or switch over to the new syscall entirely now and fall back on the old syscall if -f isn=E2=80=99t passed. Defining a new syscall = also lets you not need the name_old_syscall member in the struct, and gives you a clean, fully-extensible syscall to which future features can be added in a backwards-compatible way, rather than forever keeping around this legacy mess. Jess
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