Re: [Libguestfs] [PATCH nbdkit v2 1/3] server: Add new APIs for reading the client’s SO_PEERCRED.
On 10/5/20 9:04 AM, Richard W.M. Jones wrote:
> IIUC, gid_t/uid_t don't have their signed-ness specified by
POSIX,
> nor size, but you're required to cast negative values eg
>
> gid_t foo = (gid_t)-1;
>
> based on this, libvirt chose to expose them as "unsigned long long" to
> maximise future proofing.
We need an in-band error indication. I wonder if there are systems
with valid UID or GID == (uint64_t)-1 ?
No. POSIX chown() ensures that that ([ug]id_t)(-1) can only be used as
markers, never as valid ids. Thus they are always safe for in-band
error indication. But Dan is correct that because it is unspecified if
[ug]id_t is signed or unsigned, and also unspecified whether they are
16-, 32-, or 64-bit quantities, makes it mandatory to write the cast
(that is, you HAVE to use ((uid_t)(-1)) instead of assuming that -1 will
promote to the correct type).
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
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