Messages in this thread | | | Date | Mon, 17 Apr 2023 10:38:22 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2 1/5] fcntl: Cast commands with int args explicitly |
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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; > > break; > > 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.
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