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    SubjectRe: [PATCH 5/9] fs: remove various compat readv/writev helpers
    On Wed, Sep 23, 2020 at 12:39 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
    >
    > On Wed, Sep 23, 2020 at 03:59:01PM +0100, Al Viro wrote:
    >
    > > > That's a very good question. But it does not just compile but actually
    > > > works. Probably because all the syscall wrappers mean that we don't
    > > > actually generate the normal names. I just tried this:
    > > >
    > > > --- a/include/linux/syscalls.h
    > > > +++ b/include/linux/syscalls.h
    > > > @@ -468,7 +468,7 @@ asmlinkage long sys_lseek(unsigned int fd, off_t offset,
    > > > asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);
    > > > asmlinkage long sys_write(unsigned int fd, const char __user *buf,
    > > > size_t count);
    > > > -asmlinkage long sys_readv(unsigned long fd,
    > > > +asmlinkage long sys_readv(void *fd,
    > > >
    > > > for fun, and the compiler doesn't care either..
    > >
    > > Try to build it for sparc or ppc...
    >
    > FWIW, declarations in syscalls.h used to serve 4 purposes:
    > 1) syscall table initializers needed symbols declared
    > 2) direct calls needed the same
    > 3) catching mismatches between the declarations and definitions
    > 4) centralized list of all syscalls
    >
    > (2) has been (thankfully) reduced for some time; in any case, ksys_... is
    > used for the remaining ones.
    >
    > (1) and (3) are served by syscalls.h in architectures other than x86, arm64
    > and s390. On those 3 (1) is done otherwise (near the syscall table initializer)
    > and (3) is not done at all.
    >
    > I wonder if we should do something like
    >
    > SYSCALL_DECLARE3(readv, unsigned long, fd, const struct iovec __user *, vec,
    > unsigned long, vlen);
    > in syscalls.h instead, and not under that ifdef.
    >
    > Let it expand to declaration of sys_...() in generic case and, on x86, into
    > __do_sys_...() and __ia32_sys_...()/__x64_sys_...(), with types matching
    > what SYSCALL_DEFINE ends up using.
    >
    > Similar macro would cover compat_sys_...() declarations. That would
    > restore mismatch checking for x86 and friends. AFAICS, the cost wouldn't
    > be terribly high - cpp would have more to chew through in syscalls.h,
    > but it shouldn't be all that costly. Famous last words, of course...
    >
    > Does anybody see fundamental problems with that?

    I think this would be a good idea. I have been working on a patchset
    to clean up the conditional syscall handling (sys_ni.c), and conflicts
    with the prototypes in syscalls.h have been getting in the way.
    Having the prototypes use SYSCALL_DECLAREx(...) would solve that
    issue.

    --
    Brian Gerst

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