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Mon, 26 Jan 2026 08:05:00 -0800 (PST) Received: from localhost ([2a01:4b00:d036:ae00:53d2:28bc:8a2d:b680]) by smtp.gmail.com with UTF8SMTPSA id ffacd0b85a97d-435b1e7156dsm31892536f8f.20.2026.01.26.08.04.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Jan 2026 08:05:00 -0800 (PST) From: luca.boccassi@gmail.com To: libc-alpha@sourceware.org Cc: adhemerval.zanella@linaro.org, fweimer@redhat.com Subject: [PATCH v3] linux: use PIDFD_GET_INFO ioctl for pidfd_getpid() if available Date: Mon, 26 Jan 2026 16:03:42 +0000 Message-ID: <20260126160451.360738-1-luca.boccassi@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260124220504.709905-1-luca.boccassi@gmail.com> References: <20260124220504.709905-1-luca.boccassi@gmail.com> MIME-Version: 1.0 X-Spam-Status: No, score=-11.0 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, GIT_PATCH_0, RCVD_IN_DNSWL_BLOCKED, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.30 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: libc-alpha-bounces~patchwork=sourceware.org@sourceware.org From: Luca Boccassi Linux v6.13 introduced a new ioctl to query info from a pidfd. The advantage of this vs. parsing /proc/ is that it works even when procfs is not mounted. It's also a single syscall, and doesn't need manual string parsing. Use it when available. Signed-off-by: Luca Boccassi --- v2: use sys/pidfd.h instead of installed kernel headers v3: add __ASSUME_PIDFD_GET_INFO use atomics for support cache var sysdeps/unix/sysv/linux/kernel-features.h | 8 +++ sysdeps/unix/sysv/linux/pidfd_getpid.c | 63 ++++++++++++++++++++--- 2 files changed, 63 insertions(+), 8 deletions(-) diff --git a/sysdeps/unix/sysv/linux/kernel-features.h b/sysdeps/unix/sysv/linux/kernel-features.h index 01c865e2d5..94bdbbb730 100644 --- a/sysdeps/unix/sysv/linux/kernel-features.h +++ b/sysdeps/unix/sysv/linux/kernel-features.h @@ -269,4 +269,12 @@ # define __ASSUME_MSEAL 0 #endif +/* The PIDFD_GET_INFO ioctl was introduced across all architectures in Linux + 6.13. */ +#if __LINUX_KERNEL_VERSION >= 0x060D00 +# define __ASSUME_PIDFD_GET_INFO 1 +#else +# define __ASSUME_PIDFD_GET_INFO 0 +#endif + #endif /* kernel-features.h */ diff --git a/sysdeps/unix/sysv/linux/pidfd_getpid.c b/sysdeps/unix/sysv/linux/pidfd_getpid.c index 860829cf07..7f87330bac 100644 --- a/sysdeps/unix/sysv/linux/pidfd_getpid.c +++ b/sysdeps/unix/sysv/linux/pidfd_getpid.c @@ -23,8 +23,11 @@ #include #include #include +#include +#include #include +#if !__ASSUME_PIDFD_GET_INFO #define FDINFO_TO_FILENAME_PREFIX "/proc/self/fdinfo/" #define FDINFO_FILENAME_LEN \ @@ -91,15 +94,9 @@ parse_fdinfo (const char *l, void *arg) return 1; } -pid_t -pidfd_getpid (int fd) +static pid_t +getpid_fdinfo (int fd) { - if (__glibc_unlikely (fd < 0)) - { - __set_errno (EBADF); - return -1; - } - char fdinfoname[FDINFO_FILENAME_LEN]; char *p = mempcpy (fdinfoname, FDINFO_TO_FILENAME_PREFIX, @@ -126,3 +123,53 @@ pidfd_getpid (int fd) return fdinfo.pid; } +#endif + +static pid_t +getpid_ioctl (int fd) +{ + struct pidfd_info info = { .mask = PIDFD_INFO_PID }; + +#if !__ASSUME_PIDFD_GET_INFO + static int pidfd_get_info_supported = 1; + + if (atomic_load_relaxed (&pidfd_get_info_supported) == 0) + return -EOPNOTSUPP; +#endif + + /* New in kernel 6.13 */ + if (ioctl (fd, PIDFD_GET_INFO, &info) < 0) + { +#if !__ASSUME_PIDFD_GET_INFO + if (errno == ENOTTY || errno == EINVAL) + { + atomic_store_relaxed (&pidfd_get_info_supported, 0); + return -EOPNOTSUPP; + } +#endif + + return INLINE_SYSCALL_ERROR_RETURN_VALUE (errno); + } + + return info.pid; +} + +pid_t +pidfd_getpid (int fd) +{ + if (__glibc_unlikely (fd < 0)) + { + __set_errno (EBADF); + return -1; + } + +#if __ASSUME_PIDFD_GET_INFO + return getpid_ioctl (fd); +#else + pid_t pid = getpid_ioctl (fd); + if (pid != -EOPNOTSUPP) + return pid; + + return getpid_fdinfo (fd); +#endif +}