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Fri, 25 Sep 2026 14:16:05 -0700 (PDT) From: Danish Khateeb To: Willy Tarreau , =?UTF-8?q?Thomas=20Wei=C3=9Fschuh?= Cc: Shuah Khan , Sven Schnelle , Benjamin Berg , linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, Danish Khateeb Subject: [PATCH v2 2/3] tools/nolibc: fix FD_SET(), FD_CLR() and FD_ISSET() on 64-bit Date: Fri, 25 Sep 2026 16:15:59 -0500 Message-ID: <20260925211600.119116-3-danishkhateeb03@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260925211600.119116-1-danishkhateeb03@gmail.com> References: <20260925211600.119116-1-danishkhateeb03@gmail.com> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Since commit feaf75658783 ("nolibc: fix fd_set type"), an fd_set is an array of unsigned long, but the FD_* macros still build their masks as an int or an unsigned int: 1 << n in FD_SET(), 1U << n in FD_CLR() and FD_ISSET(), where n is the fd modulo 64 on 64-bit architectures. So: - FD_SET() of an fd with n == 31 sets the bits for n = 31-63, as 1 << 31 is negative and gets sign-extended. - For n >= 32 the shifts are undefined. x86-64, for instance, masks the shift count, so FD_SET(40) and FD_ISSET(40) use the bit of fd 8. - FD_CLR() zero-extends ~(1U << n), so it also clears the bits for n = 32-63, whichever fd it is given. select() on fd 40, for instance, fails with EBADF, or watches fd 8 instead if that one is open. Build the masks from 1UL. Fixes: feaf75658783 ("nolibc: fix fd_set type") Assisted-by: LLM Signed-off-by: Danish Khateeb --- Notes: v2: No changes except dropping Cc: stable. v1: https://lore.kernel.org/all/20260924015222.31693-3-danishkhateeb03@gmail.com/ tools/include/nolibc/sys/select.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/tools/include/nolibc/sys/select.h b/tools/include/nolibc/sys/select.h index 6d65d9ef3d6a..f299c79939f5 100644 --- a/tools/include/nolibc/sys/select.h +++ b/tools/include/nolibc/sys/select.h @@ -26,7 +26,7 @@ typedef struct { int __fd = (fd); \ if (__fd >= 0) \ __set->fds[__fd / FD_SETIDXMASK] &= \ - ~(1U << (__fd & FD_SETBITMASK)); \ + ~(1UL << (__fd & FD_SETBITMASK)); \ } while (0) #define FD_SET(fd, set) do { \ @@ -34,7 +34,7 @@ typedef struct { int __fd = (fd); \ if (__fd >= 0) \ __set->fds[__fd / FD_SETIDXMASK] |= \ - 1 << (__fd & FD_SETBITMASK); \ + 1UL << (__fd & FD_SETBITMASK); \ } while (0) #define FD_ISSET(fd, set) ({ \ @@ -43,7 +43,7 @@ typedef struct { int __r = 0; \ if (__fd >= 0) \ __r = !!(__set->fds[__fd / FD_SETIDXMASK] & \ -1U << (__fd & FD_SETBITMASK)); \ +1UL << (__fd & FD_SETBITMASK)); \ __r; \ }) -- 2.55.0