On 1/21/22 11:53 AM, Andrii Nakryiko wrote:
On Fri, Jan 21, 2022 at 11:31 AM Kenny Yu <kennyyu@xxxxxx> wrote:This adds a helper for bpf programs to read the memory of other tasks. This also adds the ability for bpf iterator programs to be sleepable. This changes `bpf_iter_run_prog` to use the appropriate synchronization for sleepable bpf programs. With sleepable bpf iterator programs, we can no longer use `rcu_read_lock()` and must use `rcu_read_lock_trace()` instead to protect the bpf program. As an example use case at Meta, we are using a bpf task iterator program and this new helper to print C++ async stack traces for all threads of a given process. Signed-off-by: Kenny Yu <kennyyu@xxxxxx> --- include/linux/bpf.h | 1 + include/uapi/linux/bpf.h | 10 ++++++++++ kernel/bpf/bpf_iter.c | 20 ++++++++++++++----- kernel/bpf/helpers.c | 35 ++++++++++++++++++++++++++++++++++ kernel/trace/bpf_trace.c | 2 ++ tools/include/uapi/linux/bpf.h | 10 ++++++++++ 6 files changed, 73 insertions(+), 5 deletions(-) diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 80e3387ea3af..5917883e528b 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h @@ -2229,6 +2229,7 @@ extern const struct bpf_func_proto bpf_kallsyms_lookup_name_proto; extern const struct bpf_func_proto bpf_find_vma_proto; extern const struct bpf_func_proto bpf_loop_proto; extern const struct bpf_func_proto bpf_strncmp_proto; +extern const struct bpf_func_proto bpf_copy_from_user_task_proto; const struct bpf_func_proto *tracing_prog_func_proto( enum bpf_func_id func_id, const struct bpf_prog *prog); diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h index fe2272defcd9..d82d9423874d 100644 --- a/include/uapi/linux/bpf.h +++ b/include/uapi/linux/bpf.h @@ -5049,6 +5049,15 @@ union bpf_attr { * This helper is currently supported by cgroup programs only. * Return * 0 on success, or a negative error in case of failure. + * + * long bpf_copy_from_user_task(void *dst, u32 size, const void *user_ptr, struct task_struct *tsk, u64 flags) + * Description + * Read *size* bytes from user space address *user_ptr* in *tsk*'s + * address space, and stores the data in *dst*. *flags* is not + * used yet and is provided for future extensibility. This helper + * can only be used by sleepable programs."On error dst buffer is zeroed out."? This is an explicit guarantee.+ * Return + * 0 on success, or a negative error in case of failure. */ #define __BPF_FUNC_MAPPER(FN) \ FN(unspec), \ @@ -5239,6 +5248,7 @@ union bpf_attr { FN(get_func_arg_cnt), \ FN(get_retval), \ FN(set_retval), \ + FN(copy_from_user_task), \ /* */ /* integer value in 'imm' field of BPF_CALL instruction selects which helper diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c index b7aef5b3416d..110029ede71e 100644 --- a/kernel/bpf/bpf_iter.c +++ b/kernel/bpf/bpf_iter.c @@ -5,6 +5,7 @@ #include <linux/anon_inodes.h> #include <linux/filter.h> #include <linux/bpf.h> +#include <linux/rcupdate_trace.h> struct bpf_iter_target_info { struct list_head list; @@ -684,11 +685,20 @@ int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx) { int ret; - rcu_read_lock(); - migrate_disable(); - ret = bpf_prog_run(prog, ctx); - migrate_enable(); - rcu_read_unlock(); + if (prog->aux->sleepable) { + rcu_read_lock_trace(); + migrate_disable(); + might_fault(); + ret = bpf_prog_run(prog, ctx); + migrate_enable(); + rcu_read_unlock_trace(); + } else { + rcu_read_lock(); + migrate_disable(); + ret = bpf_prog_run(prog, ctx); + migrate_enable(); + rcu_read_unlock(); + }I think this sleepable bpf_iter change deserves its own patch. It has nothing to do with bpf_copy_from_user_task() helper.
Without the above change, using bpf_copy_from_user_task() will trigger rcu warning and may produce incorrect result. One option is to put the above in a preparation patch before introducing bpf_copy_from_user_task() so we won't have bisecting issues.
/* bpf program can only return 0 or 1: * 0 : okay
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