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Date: Mon, 22 Apr 2024 21:39:17 -0700
From: Tony Ambardar <tony.ambardar@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH v1] add renameat2 linux syscall wrapper
On Mon, Apr 22, 2024 at 06:47:26PM -0400, Rich Felker wrote:
> On Sun, Apr 21, 2024 at 08:36:40AM -0700, Tony Ambardar wrote:
> > This syscall is available since Linux 3.15 and also implemented in glibc
> > from version 2.28. It is commonly used in filesystem or security contexts.
> >
> > Defines RENAME_NOREPLACE, RENAME_EXCHANGE, RENAME_WHITEOUT are guarded by
> > _GNU_SOURCE as with glibc.
> >
> > Signed-off-by: Tony Ambardar <Tony.Ambardar@...il.com>
> > ---
> > include/stdio.h | 7 +++++++
> > src/linux/renameat2.c | 8 ++++++++
> > 2 files changed, 15 insertions(+)
> > create mode 100644 src/linux/renameat2.c
> >
> > diff --git a/include/stdio.h b/include/stdio.h
> > index cb858618..8312c3bf 100644
> > --- a/include/stdio.h
> > +++ b/include/stdio.h
> > @@ -158,6 +158,13 @@ char *ctermid(char *);
> > #define L_ctermid 20
> > #endif
> >
> > +#if defined(_GNU_SOURCE)
> > +#define RENAME_NOREPLACE (1 << 0)
> > +#define RENAME_EXCHANGE (1 << 1)
> > +#define RENAME_WHITEOUT (1 << 2)
> > +
> > +int renameat2(int, const char *, int, const char *, unsigned int);
> > +#endif
>
> s/unsigned int/unsigned/ and maybe just write out the constants? I
> think that's the style musl uses most places.
Right, the constants-by-shift mirrored glibc and I saw these also in musl
e.g. hwcap.h, elf.h, mount.h, syslog.h, etc. I'm not wedded to either but
just wanted to be consistent. Please confirm a preference and I'll update.
>
> > #if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) \
> > || defined(_BSD_SOURCE)
> > diff --git a/src/linux/renameat2.c b/src/linux/renameat2.c
> > new file mode 100644
> > index 00000000..3062aa15
> > --- /dev/null
> > +++ b/src/linux/renameat2.c
> > @@ -0,0 +1,8 @@
> > +#define _GNU_SOURCE
> > +#include <stdio.h>
> > +#include "syscall.h"
> > +
> > +int renameat2(int oldfd, const char *old, int newfd, const char *new, unsigned int flags)
> > +{
> > + return syscall(SYS_renameat2, oldfd, old, newfd, new, flags);
> > +}
> > --
> > 2.34.1
>
> This probably at least needs to support flags==0 on kernels without
> SYS_renameat2 by calling renameat in that case. Then I'm not sure if
> ENOSYS should be kept if the new syscall is missing, or just EINVAL or
> whatever is used to report unsupported flags.
I had wondered about this too and looked at all 'arch/.../syscall.h.in'
files. All arches support SYS_renameat2, but riscv, aarch64, loongarch64
and or1k are missing SYS_rename and/or SYS_renameat. Also, current code in
stdio/rename.c and unistd/renameat.c may fall back _unconditionally_ to
use SYS_renameat2 in these cases, with the assumption it's always present.
I think this means we're OK? Or do we allow for old kernels which lack
SYS_renameat2, in which case some existing code would need updating?
> Rich
Thanks,
Tony
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