[PATCH] riscv: uapi: Lie about having futex()

[Palmer Dabbelt]

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 at 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(+)

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
-- 
2.39.0