From: Alban Crequy There are two categories of users for process_vm_readv: 1. Debuggers like GDB or strace. When a debugger attempts to read the target memory and triggers a page fault, the page fault needs to be resolved so that the debugger can accurately interpret the memory. A debugger is typically attached to a single process. 2. Profilers like OpenTelemetry eBPF Profiler. The profiler uses a perf event to get stack traces from all processes at 20Hz (20 stack traces to resolve per second). For interpreted languages (Ruby, Python, etc.), the profiler uses process_vm_readv to get the correct symbols. In this case, performance is the most important. It is fine if some stack traces cannot be resolved as long as it is not statistically significant. The current behaviour of process_vm_readv is to resolve page faults in the target VM. This is as desired for debuggers, but unwelcome for profilers because the page fault resolution could take a lot of time depending on the backing filesystem. Additionally, since profilers monitor all processes, we don't want a slow page fault resolution for one target process slowing down the monitoring for all other target processes. This patch adds the flag PROCESS_VM_NOWAIT, so the caller can choose to not block on IO if the memory access causes a page fault. When a page is not resident and would require IO to fault in, the syscall returns a short read (the number of bytes successfully read before the fault) or -1 with errno set to EFAULT if no bytes were read. Additionally, this patch adds the flag PROCESS_VM_PIDFD to refer to the remote process via PID file descriptor instead of PID. Such a file descriptor can be obtained with pidfd_open(2). This is useful to avoid the pid number being reused. It is unlikely to happen for debuggers because they can monitor the target process termination in other ways (ptrace), but can be helpful in some profiling scenarios. When using PROCESS_VM_PIDFD, the first argument is a pidfd instead of a pid. If the pidfd is invalid, the syscall returns -1 with errno set to EBADF. If a given flag is unsupported, the syscall returns the error EINVAL without checking the buffers. This gives a way to userspace to detect whether the current kernel supports a specific flag: process_vm_readv(pid, NULL, 1, NULL, 1, PROCESS_VM_PIDFD) -> EINVAL if the kernel does not support the flag PROCESS_VM_PIDFD (before this patch) -> EFAULT if the kernel supports the flag (after this patch) Suggested man page update for process_vm_readv(2): The flags argument is the bitwise OR of zero or more of these flags: PROCESS_VM_PIDFD (since Linux 7.x) The pid argument is a PID file descriptor (see pidfd_open(2)) instead of a PID number. When using this flag, the existing ESRCH error applies if the process referred to by the pidfd has exited. PROCESS_VM_NOWAIT (since Linux 7.x) Do not block on IO. If a page in the remote address space is not resident and would require disk IO to fault in, the system call returns a short read or fails with EFAULT if no bytes were read. Additional error: EBADF pid is not a valid file descriptor (PROCESS_VM_PIDFD only). Signed-off-by: Alban Crequy --- v4: - Rename process_vm.h to process_vm_access.h (David Hildenbrand) - Fix MAINTAINERS alphabetical sort order (David Hildenbrand) - Document NOWAIT return value and PIDFD EBADF error (David Hildenbrand) - Add suggested man page update text (David Hildenbrand, Christian Brauner, Mike Rapoport) - Keep (1UL << N) in UAPI header (David Hildenbrand) v3: - Fix ERR_PTR handling for pidfd_get_task(): use IS_ERR()/PTR_ERR() for the pidfd path, matching process_madvise() (Usama Arif, Sashiko) v2: - Expand commit message with use-case motivation (David Hildenbrand) - Use unsigned long consistently for pvm_flags parameter (David Hildenbrand) - Add PROCESS_VM_SUPPORTED_FLAGS kernel-internal define (David Hildenbrand) - Keep (1UL << N) in UAPI header: BIT() is defined in vdso/bits.h which is not exported to userspace, so UAPI headers using BIT() would break when included from userspace programs (David Hildenbrand) MAINTAINERS | 1 + include/uapi/linux/process_vm_access.h | 9 +++++++ mm/process_vm_access.c | 34 +++++++++++++++++++------- 3 files changed, 35 insertions(+), 9 deletions(-) create mode 100644 include/uapi/linux/process_vm_access.h diff --git a/MAINTAINERS b/MAINTAINERS index 7492fefa447c..e64252164191 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -16788,6 +16788,7 @@ F: include/linux/pgtable.h F: include/linux/ptdump.h F: include/linux/vmpressure.h F: include/linux/vmstat.h +F: include/uapi/linux/process_vm_access.h F: fs/proc/meminfo.c F: kernel/fork.c F: mm/Kconfig diff --git a/include/uapi/linux/process_vm_access.h b/include/uapi/linux/process_vm_access.h new file mode 100644 index 000000000000..2196c9c46351 --- /dev/null +++ b/include/uapi/linux/process_vm_access.h @@ -0,0 +1,9 @@ +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ +#ifndef _UAPI_LINUX_PROCESS_VM_ACCESS_H +#define _UAPI_LINUX_PROCESS_VM_ACCESS_H + +/* Flags for process_vm_readv/process_vm_writev */ +#define PROCESS_VM_PIDFD (1UL << 0) +#define PROCESS_VM_NOWAIT (1UL << 1) + +#endif /* _UAPI_LINUX_PROCESS_VM_ACCESS_H */ diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c index 656d3e88755b..31004dd3c9e3 100644 --- a/mm/process_vm_access.c +++ b/mm/process_vm_access.c @@ -14,6 +14,9 @@ #include #include #include +#include + +#define PROCESS_VM_SUPPORTED_FLAGS (PROCESS_VM_PIDFD | PROCESS_VM_NOWAIT) /** * process_vm_rw_pages - read/write pages from task specified @@ -68,6 +71,7 @@ static int process_vm_rw_pages(struct page **pages, * @mm: mm for task * @task: task to read/write from * @vm_write: 0 means copy from, 1 means copy to + * @pvm_flags: PROCESS_VM_* flags * Returns 0 on success or on failure error code */ static int process_vm_rw_single_vec(unsigned long addr, @@ -76,7 +80,8 @@ static int process_vm_rw_single_vec(unsigned long addr, struct page **process_pages, struct mm_struct *mm, struct task_struct *task, - int vm_write) + int vm_write, + unsigned long pvm_flags) { unsigned long pa = addr & PAGE_MASK; unsigned long start_offset = addr - pa; @@ -91,6 +96,8 @@ static int process_vm_rw_single_vec(unsigned long addr, if (vm_write) flags |= FOLL_WRITE; + if (pvm_flags & PROCESS_VM_NOWAIT) + flags |= FOLL_NOWAIT; while (!rc && nr_pages && iov_iter_count(iter)) { int pinned_pages = min_t(unsigned long, nr_pages, PVM_MAX_USER_PAGES); @@ -141,7 +148,7 @@ static int process_vm_rw_single_vec(unsigned long addr, * @iter: where to copy to/from locally * @rvec: iovec array specifying where to copy to/from in the other process * @riovcnt: size of rvec array - * @flags: currently unused + * @flags: process_vm_readv/writev flags * @vm_write: 0 if reading from other process, 1 if writing to other process * * Returns the number of bytes read/written or error code. May @@ -163,6 +170,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, unsigned long nr_pages_iov; ssize_t iov_len; size_t total_len = iov_iter_count(iter); + unsigned int f_flags; /* * Work out how many pages of struct pages we're going to need @@ -194,10 +202,18 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, } /* Get process information */ - task = find_get_task_by_vpid(pid); - if (!task) { - rc = -ESRCH; - goto free_proc_pages; + if (flags & PROCESS_VM_PIDFD) { + task = pidfd_get_task(pid, &f_flags); + if (IS_ERR(task)) { + rc = PTR_ERR(task); + goto free_proc_pages; + } + } else { + task = find_get_task_by_vpid(pid); + if (!task) { + rc = -ESRCH; + goto free_proc_pages; + } } mm = mm_access(task, PTRACE_MODE_ATTACH_REALCREDS); @@ -215,7 +231,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, for (i = 0; i < riovcnt && iov_iter_count(iter) && !rc; i++) rc = process_vm_rw_single_vec( (unsigned long)rvec[i].iov_base, rvec[i].iov_len, - iter, process_pages, mm, task, vm_write); + iter, process_pages, mm, task, vm_write, flags); /* copied = space before - space after */ total_len -= iov_iter_count(iter); @@ -244,7 +260,7 @@ static ssize_t process_vm_rw_core(pid_t pid, struct iov_iter *iter, * @liovcnt: size of lvec array * @rvec: iovec array specifying where to copy to/from in the other process * @riovcnt: size of rvec array - * @flags: currently unused + * @flags: process_vm_readv/writev flags * @vm_write: 0 if reading from other process, 1 if writing to other process * * Returns the number of bytes read/written or error code. May @@ -266,7 +282,7 @@ static ssize_t process_vm_rw(pid_t pid, ssize_t rc; int dir = vm_write ? ITER_SOURCE : ITER_DEST; - if (flags != 0) + if (flags & ~PROCESS_VM_SUPPORTED_FLAGS) return -EINVAL; /* Check iovecs */ -- 2.45.0