Am Montag, dem 18.09.2023 um 10:47 +0200 schrieb Richard Biener via Gcc: > On Mon, Sep 18, 2023 at 10:17 AM Martin Uecker <ma.uec...@gmail.com> wrote: > > > > Am Montag, dem 18.09.2023 um 09:31 +0200 schrieb Richard Biener via Gcc: > > > On Sat, Sep 16, 2023 at 10:38 AM Martin Uecker via Gcc <gcc@gcc.gnu.org> > > > wrote: > > > > > > > > > > > > > > > > (moved to gcc@) > > > > > > > > > On Fri, Sep 15, 2023 at 08:18:28AM -0700, Andrew Pinski wrote: > > > > > > On Fri, Sep 15, 2023 at 8:12 AM Qing Zhao <qing.z...@oracle.com> > > > > > > wrote: > > > > > > > > > > > > > > > > > > > > > > > > > > > > > On Sep 15, 2023, at 3:43 AM, Xi Ruoyao <xry...@xry111.site> > > > > > > > > wrote: > > > > > > > > > > > > > > > > On Thu, 2023-09-14 at 21:41 +0000, Qing Zhao wrote: > > > > > > > > > > > CLANG already provided > > > > > > > > > > > -fsanitize=unsigned-integer-overflow. GCC > > > > > > > > > > > might need to do the same. > > > > > > > > > > > > > > > > > > > > NO. There is no such thing as unsigned integer overflow. > > > > > > > > > > That option > > > > > > > > > > is badly designed and the GCC community has rejected a few > > > > > > > > > > times now > > > > > > > > > > having that sanitizer before. It is bad form to have a > > > > > > > > > > sanitizer for > > > > > > > > > > well defined code. > > > > > > > > > > > > > > > > > > Even though unsigned integer overflow is well defined, it > > > > > > > > > might be > > > > > > > > > unintentional, shall we warn user about this? > > > > > > > > > > > > > > > > *Everything* could be unintentional and should be warned then. > > > > > > > > GCC is a > > > > > > > > compiler, not an advanced AI educating the programmers. > > > > > > > > > > > > > > Well, you are right in some sense. -:) > > > > > > > > > > > > > > However, overflow is one important source for security flaws, > > > > > > > it’s important for compilers to detect > > > > > > > overflows in the programs in general. > > > > > > > > > > > > Except it is NOT an overflow. Rather it is wrapping. That is a big > > > > > > point here. unsigned wraps and does NOT overflow. Yes there is a > > > > > > major > > > > > > difference. > > > > > > > > > > Right, yes. I will try to pick my language very carefully. :) > > > > > > > > > > The practical problem I am trying to solve in the 30 million lines of > > > > > Linux kernel code is that of catching arithmetic wrap-around. The > > > > > problem is one of evolving the code -- I can't just drop -fwrapv and > > > > > -fwrapv-pointer because it's not possible to fix all the cases at > > > > > once. > > > > > (And we really don't want to reintroduce undefined behavior.) > > > > > > > > > > So, for signed, pointer, and unsigned types, we need: > > > > > > > > > > a) No arithmetic UB -- everything needs to have deterministic > > > > > behavior. > > > > > The current solution here is "-fno-strict-overflow", which > > > > > eliminates > > > > > the UB and makes sure everything wraps. > > > > > > > > > > b) A way to run-time warn/trap on overflow/underflow/wrap-around. This > > > > > would work with -fsanitize=[signed-integer|pointer]-overflow except > > > > > due to "a)" we always wrap. And there isn't currently coverage like > > > > > this for unsigned (in GCC). > > > > > > > > > > Our problem is that the kernel is filled with a mix of places where > > > > > there > > > > > is intended wrap-around and unintended wrap-around. We can chip away > > > > > at > > > > > fixing the intended wrap-around that we can find with static > > > > > analyzers, > > > > > etc, but at the end of the day there is a long tail of finding the > > > > > places > > > > > where intended wrap-around is hiding. But when the refactoring is > > > > > sufficiently completely, we can move the wrap-around warning to a > > > > > trap, > > > > > and the kernel will not longer have this class of security flaw. > > > > > > > > > > As a real-world example, here is a bug where a u8 wraps around causing > > > > > an under-allocation that allowed for a heap overwrite: > > > > > > > > > > https://git.kernel.org/linus/6311071a0562 > > > > > https://elixir.bootlin.com/linux/v6.5/source/net/wireless/nl80211.c#L5422 > > > > > > > > > > If there were more than 255 elements in a linked list, the allocation > > > > > would be too small, and the second loop would write past the end of > > > > > the > > > > > allocation. This is a pretty classic allocation underflow and linear > > > > > heap write overflow security flaw. (And it would be trivially stopped > > > > > by > > > > > trapping on the u8 wrap around.) > > > > > > > > > > So, I want to be able to catch that at run-time. But we also have code > > > > > doing things like "if (ulong + offset < ulong) { ... }": > > > > > > > > > > https://elixir.bootlin.com/linux/v6.5/source/drivers/crypto/axis/artpec6_crypto.c#L1187 > > > > > > > > > > This is easy for a static analyzer to find and we can replace it with > > > > > a > > > > > non-wrapping test (e.g. __builtin_add_overflow()), but we'll not find > > > > > them all immediately, especially for the signed and pointer cases. > > > > > > > > > > So, I need to retain the "everything wraps" behavior while still being > > > > > able to detect when it happens. > > > > > > > > > > > > Hi Kees, > > > > > > > > I have a couple of questions: > > > > > > > > Currently, my thinking was that you would use signed integers > > > > if you want the usual integer arithmetic rules we know from > > > > elementary school and if you overflow this is clearly a bug > > > > you can diagnose with UBsan. > > > > > > > > There are people who think that signed overflow should be > > > > defined to wrap, but I think this would be a severe > > > > mistake because then code would start to rely on it, which > > > > makes it then difficult to differentiate between bugs and > > > > intended uses (e.g. the unfortunate situation you have > > > > with the kernel). > > > > > > > > I assume you want to combine UBSan plus wrapping for > > > > production use? Or only for testing? Or in other words: > > > > why would testing UBSan and production with wrapping > > > > not be sufficient to find and fix all bugs? > > > > > > > > Wrapping would not be correct because it may lead to > > > > logic errors or use-after-free etc. I assume it is still > > > > preferred because it more deterministic than whatever comes > > > > out of the optimizer assuming that overflow has UB. Is this > > > > the reasoning applied here? > > > > > > > > > > > > For unsigned the intended use case is modulo arithmetic > > > > where wrapping is the correct behavior. At least, this > > > > is what I thought so far.. This seems also to be the > > > > position of the overall GCC community rejecting > > > > -fsanitize=unsigned-integer-overflow. > > > > > > > > But then there are also people like Richard Seacord that > > > > have the position that one should use "unsigned" for > > > > every quantity which can not be negative, which implies > > > > that then the modulo use case becomes the exception. > > > > > > > > Related to this I have the following question: In the > > > > bug you refer to above, an unsigned variable was used > > > > for something that is not meant for modulo arithmetic. > > > > Is this done in the kernel to save space? I.e. because > > > > 127 would not be enough as maximum but going to i16 takes > > > > to much space? or is this for compatibility with some > > > > on-the-wire protocol? > > > > > > > > Would an attribute for variables help that tells the > > > > compiler that if stores to the variable wrap around > > > > then this is not intended and this is an error? > > > > > > > > u8 x [[nowrap]]; > > > > > > > > x = 256; // can be diagnosed ? > > > > > > There's a problem with representing this, it's a long-standing thing > > > standing > > > in the way of handling -fwrapv in the IL. Maybe C wants to have > > > 'nonnegative int' in addition to 'unsigned int', or 'bit int' for the > > > "just some two-complement bits". > > > > > > For GCC we think of -fwrapv to perform signed integer arithmetic as if > > > promoted to a large enough type first and then truncated to the original > > > type in the implementation defined manner (modulo reducing). But we > > > end up doing what the actual hardware does (I'm for example unsure > > > how that handles INT_MIN / -1 - x86 documents it as raising #DE > > > which it indeed does, even with -fwrapv). > > > > > > volatile int a, b; > > > int main () > > > { > > > a = -__INT_MAX__ - 1; > > > b = -1; > > > int z = a / b; > > > __builtin_printf ("%d\n", z); > > > } > > > > > > raises SIGFPE with and without -fwrapv. I'll note that we document > > > -fwrapv to only affect addition, subtraction and multiplication > > > (it also affects negation), matching the libgcc *v functions we have > > > for -ftrapv. -fsanitize=undefined instruments division but > > > -fsanitize=recover doesn't help. Maybe there's a bug about this > > > recorded. The documentation of -fwrapv should likely be clarified.
Thank you for the explanation. But note that I suggested the opposite: "nowrap" . A compiler would not need to be changed at all and can simply ignore it this attribute. But if it sees that a too large value is stored in an unsigned type that is too small, it could warn in the frontend. And we could add a sanitizer mode to diagnose wraparound at run-time. (which would then not have the problem with false positives as -funsigned-integer-overflow would have) > > > > > > For recent work I failed to spot the C standards language on > > > singed overflow behavior for negation, addition, subtraction > > > and multiplication. I can find it for division though. Can someone > > > point me to where that is (moved?)? > > > > I don't think it moved. It should follow from the general rule 6.5p5: > > > > "If an exceptional condition occurs during the evaluation of an > > expression (that is, if the result is not mathematically defined > > or not in the range of representable values for its type), the > > behavior is undefined." > > Ah, and the unsigned integer type exception is then 6.2.5/9 > but using "can never overflow" wording rather than "in the range > of representable values for its type". > The "can never overflow" is consequence of modulo arithmetic, because then the result is in the representable range. > Integer "overflow" isn't defined anywhere and It might be defined in ISO/IEC 2382. > I suppose the unsigned exception also > applies to underflow for expressions like 0 - 1. Yes, because the operation is performed modulo 2^N. > > It would have been useful to repeat some of this in the sections > documenting the relevant operators. I agree. Martin
