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From:   Zhangjin Wu <falcon@tinylab.org>
To:     arnd@arndb.de
Cc:     falcon@tinylab.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, linux-riscv@lists.infradead.org,
        thomas@t-8ch.de, w@1wt.eu
Subject: Re: [PATCH v2 07/13] tools/nolibc: sys_lseek: add pure 64bit lseek
Date:   Tue, 30 May 2023 21:54:33 +0800
Message-Id: <20230530135433.405051-1-falcon@tinylab.org>
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Hi, Arnd, Willy

> On Mon, May 29, 2023, at 21:54, Zhangjin Wu wrote:
> > use sys_llseek instead of sys_lseek to add 64bit seek even in 32bit
> > platforms.
> >
> > This code is based on sysdeps/unix/sysv/linux/lseek.c of glibc and
> > src/unistd/lseek.c of musl.
> >
> > Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
> > Signed-off-by: Willy Tarreau <w@1wt.eu>
> > ---
> >  tools/include/nolibc/sys.h | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/tools/include/nolibc/sys.h b/tools/include/nolibc/sys.h
> > index 98cfa2f6d021..d0720af84b6d 100644
> > --- a/tools/include/nolibc/sys.h
> > +++ b/tools/include/nolibc/sys.h
> > @@ -672,7 +672,17 @@ int link(const char *old, const char *new)
> >  static __attribute__((unused))
> >  off_t sys_lseek(int fd, off_t offset, int whence)
> >  {
> > +#if defined(__NR_llseek) || defined(__NR__llseek)
> > +#ifndef __NR__llseek
> > +#define __NR__llseek __NR_llseek
> > +#endif
> > +	off_t result;
> > +	return my_syscall5(__NR__llseek, fd, offset >> 32, offset, &result, 
> > whence) ?: result;
> > +#elif defined(__NR_lseek)
> >  	return my_syscall3(__NR_lseek, fd, offset, whence);
> > +#else
> > +#error None of __NR_lseek, __NR_llseek nor __NR__llseek defined, 
> > cannot implement sys_lseek()
> > +#endif
> >  }
> 
> This is not technically wrong, but I think a different approach
> would be clearer: Instead of having a sys_lseek() that works
> differently depending on the macros, why not define the low-level
> helpers to match the kernel arguments like
> 
> static inline __attribute__((unused))
> __kernel_loff_t sys_lseek(int fd, __kernel_loff_t offset, int whence)
> {
> #ifdef __NR__llseek
> 	__kernel_loff_t result;
> 	return my_syscall5(__NR__llseek, fd, offset >> 32, offset, &result,  whence) ?: result;
> #else
>         
> #endif
> }
> 
> static inline __attribute__((unused))
> __kernel_off_t sys_lseek(int fd, __kernel_off_t offset, int whence)
> {
> #ifdef __NR_lseek
> 	return my_syscall3(__NR_lseek, fd, offset, whence);
> #else
>         return -ENOSYS;
> #endif
> }
> 
> And then do the selection inside of the actual lseek,
> something like
> 
> static __attribute__((unused))
> off_t lseek(int fd, off_t offset, int whence)
> {
>         off_t ret = -ENOSYS;
> 
>         if (BITS_PER_LONG == 32)
>                ret = sys_llseek(fd, offset, whence);
> 
>         if (ret == -ENOSYS)
>                ret = sys_lseek(fd, offset, whence);
> 
>         if (ret < 0) {
>                 SET_ERRNO(-ret);
>                 ret = -1;
>         }
>         return ret;
>        
> }

Yes, It is clearer, thanks. will learn carefully about the kernel types.

> 
> For the loff_t selection, there is no real need to handle the
> fallback, so this could just be an if()/else to select 32-bit
> or 64-bit, but for the time_t ones the fallback is required
> for pre-5.6 kernels.
>

Ok, will test it on the pre-5.6 versions too.

Hi, Willy, what's your suggestion about the oldest kernel versions we plan to support? ;-)

Best regards,
Zhangjin

>        Arnd