On Tue, May 23, 2023 at 04:25:45PM +0300, Alexey Izbyshev wrote: > On 2023-05-23 16:07, Catalin Marinas wrote: > > On Tue, May 23, 2023 at 11:12:37AM +0200, David Hildenbrand wrote: > > > Also, how is passing "0"s to e.g., PR_GET_THP_DISABLE reliable? We > > > need arg2 > > > -> arg5 to be 0. But wouldn't the following also just pass a 0 "int" ? > > > > > > prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0) > > > > > > I'm easily confused by such (va_args) things, so sorry for the dummy > > > questions. > > > > Isn't the prctl() prototype in the user headers defined with the first > > argument as int while the rest as unsigned long? At least from the man > > page: > > > > int prctl(int option, unsigned long arg2, unsigned long arg3, > > unsigned long arg4, unsigned long arg5); > > > > So there are no va_args tricks (which confuse me as well). > > > I have explicitly mentioned the problem with man pages in my response to > David[1]. Quoting myself: > > > This stuff *is* confusing, and note that Linux man pages don't even tell > that prctl() is actually declared as a variadic function (and for > ptrace() this is mentioned only in the notes, but not in its signature). Ah, thanks for the clarification (I somehow missed your reply). > The reality: > > * glibc: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sys/prctl.h;h=821aeefc1339b35210e8918ecfe9833ed2792626;hb=glibc-2.37#l42 > > * musl: > https://git.musl-libc.org/cgit/musl/tree/include/sys/prctl.h?h=v1.2.4#n180 > > Though there is a test in the kernel that does define its own prototype, > avoiding the issue: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/sched/cs_prctl_test.c?h=v6.3#n77 At least for glibc, it seems that there is a conversion to unsigned long: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/prctl.c#l28 unsigned long int arg2 = va_arg (arg, unsigned long int); (does va_arg expand to an actual cast?) If the libc passes a 32-bit to a kernel ABI that expects 64-bit, I think it's a user-space bug and not a kernel ABI issue. -- Catalin
