On Fri, Apr 14, 2023 at 04:46:31PM +0100, Al Viro wrote:
On Fri, Apr 14, 2023 at 04:24:55PM +0100, Luca Vizzarro wrote:
void __user *argp = (void __user *)arg;
- int argi = (int)arg;
Strictly speaking, conversion from unsigned long to int is an undefined behaviour, unless the value fits into the range representable by int ;-)
case F_SETFD: err = 0;
set_close_on_exec(fd, arg & FD_CLOEXEC);
set_close_on_exec(fd, argi & FD_CLOEXEC);Why?
case F_SETSIG: /* arg == 0 restores default behaviour. */
if (!valid_signal(arg)) {
if (!valid_signal(argi)) {Why???
break; } err = 0;
filp->f_owner.signum = arg;
filp->f_owner.signum = argi;These two are clearly bogus and I'd like to see more details on the series rationale, please.
I agree the first isn't necessary, but I don't think the second is bogus, since valid_signal() takes an unsigned long and the man page for F_SETSIG says that the argument is an int:
https://man7.org/linux/man-pages/man2/fcntl.2.html
... though arguably that could be a bug in the man page.
The cover letter really should have quoted the description that Szabolcs wote at:
https://lore.kernel.org/linux-api/Y1%2FDS6uoWP7OSkmd@arm.com/
The gist being that where the calling convention leaves narrowing to callees (as is the case on arm64 with our "AAPCS64" calling convention), if the caller passes a type which is narrower than a register, the upper bits of that register may contain junk.
So e.g. for F_SETSIG, if the userspace will try to pass some 32-bit value, leaving bits 63:32 of the argument register containing arbitrary junk. Then here we interprert the value as an unsigned long, considering that junk as part of the argument. Then valid_signal(arg) may end up rejecting the argument due to the junk uper bits, which is surprising to the caller as from its PoV it passed a 32-bit value in the correct way.
So either:
* That's a documentation bug, and userspce needs to treat the agument to F_SETSIG as an unsigned long.
* The kernel needs to narrow the argument to an int (if required by the calling convention) to prevent that.
Does that make sense, or have I missed the point you were making?
Thanks, Mark.