[Adding libc-alpha@ for some doubts]
Hi!
On Sun, May 26, 2024 at 01:27:43PM GMT, Alejandro Colomar wrote:
> On Sun, May 26, 2024 at 01:07:24PM GMT, Alejandro Colomar wrote:
> > I'm considering making sashimi of prctl(2), similar to what I did
> > recently to proc(5). Another precedent is in ioctl(2).
I'll call the pages with names such as PR_CAP_AMBIENT(2const) and
PR_CAP_AMBIENT_RAISE(2const).
While doing that, I changed the prototypes in the SYNOPSIS to things
like
int prctl(PR_CAP_AMBIENT, unsigned long op, ...);
and
int prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, unsigned long cap, 0, 0);
Which makes me wonder. glibc implements prctl(2) as a variadic
function, so those 0s are actually of type (and more importantly of
width) 'int'. This means a user passing 0 is leaving some parameters
uninitialized.
From what I can see, glibc does no magic to set unspecified parameters
to 0, so this means passing '0' results in Undefined Behavior.
I guess I should document these as 0L in the SYNOPSIS.
int prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, unsigned long cap, 0L, 0L);
All of the software I've seen out there using prctl(2) either pass 0 (as
the manual page had been suggesting), such as in shadow:
<https://github.com/shadow-maint/shadow/blob/71e28359d12491727b2e94c71d2e1e1682d45a02/lib/idmapping.c#L161>
if (prctl(PR_SET_KEEPCAPS, 1, 0, 0, 0) < 0) {
or don't pass anything at all (coreutils does this):
<https://git.savannah.gnu.org/cgit/coreutils.git/tree/src/timeout.c#n449>
if (prctl (PR_SET_DUMPABLE, 0) == 0)
Am I missing something or are all of those calls buggy?
Some prctl(2) calls report EINVAL when the unused arguments are nonzero,
while others simply ignore it, so maybe I can document the ones ignoring
the unused arguments as shorter calls:
int prctl(PR_SET_DUMPABLE, unsigned long dumpable);
And document the ones that report errors as using 0L:
int prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, unsigned long cap, 0, 0);
(BTW, util-linux seems to have this one wrong:)
<https://sources.debian.org/src/util-linux/2.40.1-2/lib/caputils.c/?hl=123#L123>
&& prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, cap, 0, 0) < 0)
What do you think about this?
Have a lovely day!
Alex
--
<https://www.alejandro-colomar.es/>
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