| Message ID | 20230119193924.21186-1-palmer@rivosinc.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | riscv: uapi: Lie about having futex() | expand |
| Context | Check | Description |
|---|---|---|
| conchuod/cover_letter | success | Single patches do not need cover letters |
| conchuod/tree_selection | success | Guessed tree name to be for-next |
| conchuod/fixes_present | success | Fixes tag not required for -next series |
| conchuod/maintainers_pattern | success | MAINTAINERS pattern errors before the patch: 13 and now 13 |
| conchuod/verify_signedoff | success | Signed-off-by tag matches author and committer |
| conchuod/kdoc | success | Errors and warnings before: 0 this patch: 0 |
| conchuod/module_param | success | Was 0 now: 0 |
| conchuod/build_rv64_gcc_allmodconfig | success | Errors and warnings before: 2025 this patch: 2025 |
| conchuod/alphanumeric_selects | success | Out of order selects before the patch: 57 and now 57 |
| conchuod/build_rv32_defconfig | success | Build OK |
| conchuod/dtb_warn_rv64 | success | Errors and warnings before: 2 this patch: 2 |
| conchuod/header_inline | success | No static functions without inline keyword in header files |
| conchuod/checkpatch | success | total: 0 errors, 0 warnings, 0 checks, 11 lines checked |
| conchuod/source_inline | success | Was 0 now: 0 |
| conchuod/build_rv64_nommu_k210_defconfig | success | Build OK |
| conchuod/verify_fixes | success | No Fixes tag |
| conchuod/build_rv64_nommu_virt_defconfig | success | Build OK |
On Jan 19 2023, Palmer Dabbelt wrote: > diff --git a/arch/riscv/include/asm/unistd.h b/arch/riscv/include/asm/unistd.h > index 221630bdbd07..26116686690a 100644 > --- a/arch/riscv/include/asm/unistd.h > +++ b/arch/riscv/include/asm/unistd.h > @@ -24,3 +24,11 @@ > #include <uapi/asm/unistd.h> > > #define NR_syscalls (__NR_syscalls) > + > +/* > + * Userspace (at least libstdc++) has come to expect SYS_futex, but we only > + * have SYS_futex_time64 on rv32. So just lie. > + */ > +#ifndef __LP64__ > +#define __NR_futex __NR_futex_time64 > +#endif This is not a uapi file, so why does this have any influence on libstdc++?
On Thu, Jan 19, 2023, at 20:39, Palmer Dabbelt wrote: > Without this libstdc++ correctly detects the lack of a futex() syscall > on rv32 and uses a fallback that doesn't work because it depends on > 64-bit atomics. > > Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com> > > --- > I'm not exactly sure why this is triggering now, as it seems like all > the conditions have been there for a while, but I had to swizzle around > the toolchain tuples recently due to some glibc build system > improvements and I suspect it's related. > > It looks viable to update libstdc++ to understand futex_time64, but > given how common SYS_futex is these days I'm guessing some other bits of > userspace will end up broken as well. This certainly feels dirty, but > it's easy. Aside from what Andreas said, this is just wrong, riscv definitely does not have a futex syscall and redirecting it to a different syscall will lead to worse bugs because of mismatched arguments. One possibility that we had discussed in the past is to actually add support for a subset of futex() in configurations without CONFIG_COMPAT_32BIT, which would include all rv32 kernels. To avoid having to define an ABI with the incompatible 32-bit timespec, this partial futex call would either have to reject all commands that take a timeout (FUTEX_WAIT, FUTEX_LOCK_PI, FUTEX_LOCK_PI2, FUTEX_WAIT_BITSET, FUTEX_WAIT_REQUEUE_PI) or reject those calls that set a non-NULL timeout pointer. I'm not convinced that this would actually be better than what we have today though: right now any application that tries to use futex_time32 but does not know futex_time64 will fail to build on riscv32. If we provide a __NR_futex, then all of these will build, but anything that actually uses a timeout will receive -ENOSYS or similar and break at runtime. Arnd
diff --git a/arch/riscv/include/asm/unistd.h b/arch/riscv/include/asm/unistd.h index 221630bdbd07..26116686690a 100644 --- a/arch/riscv/include/asm/unistd.h +++ b/arch/riscv/include/asm/unistd.h @@ -24,3 +24,11 @@ #include <uapi/asm/unistd.h> #define NR_syscalls (__NR_syscalls) + +/* + * Userspace (at least libstdc++) has come to expect SYS_futex, but we only + * have SYS_futex_time64 on rv32. So just lie. + */ +#ifndef __LP64__ +#define __NR_futex __NR_futex_time64 +#endif
Without this libstdc++ correctly detects the lack of a futex() syscall on rv32 and uses a fallback that doesn't work because it depends on 64-bit atomics. Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com> --- I'm not exactly sure why this is triggering now, as it seems like all the conditions have been there for a while, but I had to swizzle around the toolchain tuples recently due to some glibc build system improvements and I suspect it's related. It looks viable to update libstdc++ to understand futex_time64, but given how common SYS_futex is these days I'm guessing some other bits of userspace will end up broken as well. This certainly feels dirty, but it's easy. --- arch/riscv/include/asm/unistd.h | 8 ++++++++ 1 file changed, 8 insertions(+)