Segher Boessenkool <seg...@kernel.crashing.org> writes: > On Tue, Aug 31, 2021 at 11:27:20PM +1000, Michael Ellerman wrote: >> Nathan filed an LLVM bug [2], in which Eli Friedman explained that "if >> you pass a value of a type that's narrower than a register to an inline >> asm, the high bits are undefined". In this case we are passing a bool >> to the inline asm, which is a single bit value, and so the compiler >> thinks it can leave the high bits of r30 unmasked, because only the >> value of bit 0 matters. >> >> Because the inline asm compares the full width of the register (32 or >> 64-bit) we need to ensure the value passed to the inline asm has all >> bits defined. So fix it by casting to long. >> >> We also already cast to long for the similar case in BUG_ENTRY(), which >> it turns out was added to fix a similar bug in 2005 in commit >> 32818c2eb6b8 ("[PATCH] ppc64: Fix issue with gcc 4.0 compiled kernels"). > > That points to <https://gcc.gnu.org/PR23422>, which shows the correct > explanation.
That's a pity because I don't understand that explanation ^_^ Why does sign extension matter when we're comparing against zero? > The code as it was did **not** pass a bool. It perhaps passed an int > (so many macros in play, it is hard to tell for sure, but it is int or > long int, perhaps unsigned (which does not change anything here). I don't understand that. It definitely is passing a bool at the source level. Are you saying it's getting promoted somehow? It expands to: asm goto( "1: " "tdnei" " " " % 4, 0 " "\n " ".section __ex_table,\"a\";" " " ".balign 4;" " " ".long (1b) - . ;" " " ".long (%l[__label_warn_on]) - . ;" " " ".previous" " " ".section __bug_table,\"aw\"\n" "2:\t.4byte 1b - 2b, %0 - 2b\n" "\t.short %1, %2\n" ".org 2b+%3\n" ".previous\n" : : "i"("lib/klist.c"), "i"(62), "i"((1 << 0) | ((9) << 8)), "i"(sizeof(struct bug_entry)), "r"(knode_dead(knode)) ^^^^^^^^^^^^^^^^^^^^^ : : __label_warn_on); And knode_dead() returns bool: static bool knode_dead(struct klist_node *knode) { return (unsigned long)knode->n_klist & KNODE_DEAD; } So in my book that means the type there is bool. But I'm not a compiler guy so I guessing I'm missing something. > But td wants a 64-bit register, not a 32-bit one (which are the only two > possibilities for the "r" constraint on PowerPC). The cast to "long" is > fine for powerpc64, but it has nothing to do with "narrower than a > register". If it's 32-bit vs 64-bit, and the clang explanation is correct, then we'd expect the low 32-bits of the value passed to the asm to have the correct value, ie. have been masked with KNODE_DEAD. > If this is not the correct explanation for LLVM, it needs to solve some > other bug. OK. I just need to get this fixed in the kernel, so I'll do a new version with a changelog that is ~= "shrug not sure what's going on" and merge that. Then we can argue about what is really going on later :) cheeers