bpf: dispatch to sleepable file dynptr

File dynptr reads may sleep when the requested folios are not in
the page cache. To avoid sleeping in non-sleepable contexts while still
supporting valid sleepable use, given that dynptrs are non-sleepable by
default, enable sleeping only when bpf_dynptr_from_file() is invoked
from a sleepable context.

This change:
  * Introduces a sleepable constructor: bpf_dynptr_from_file_sleepable()
  * Override non-sleepable constructor with sleepable if it's always
  called in sleepable context

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: mykyta5

include/linux/bpf.h

@@ -670,6 +670,9 @@ static inline bool bpf_map_has_internal_structs(struct bpf_map *map)
 
 void bpf_map_free_internal_structs(struct bpf_map *map, void *obj);
 
+int bpf_dynptr_from_file_sleepable(struct file *file, u32 flags,
+				   struct bpf_dynptr *ptr__uninit);
+
 extern const struct bpf_map_ops bpf_map_offload_ops;
 
 /* bpf_type_flag contains a set of flags that are applicable to the values of

kernel/bpf/helpers.c

@@ -4336,6 +4336,11 @@ __bpf_kfunc int bpf_dynptr_from_file(struct file *file, u32 flags, struct bpf_dy
 	return make_file_dynptr(file, flags, false, (struct bpf_dynptr_kern *)ptr__uninit);
 }
 
+int bpf_dynptr_from_file_sleepable(struct file *file, u32 flags, struct bpf_dynptr *ptr__uninit)
+{
+	return make_file_dynptr(file, flags, true, (struct bpf_dynptr_kern *)ptr__uninit);
+}
+
 __bpf_kfunc int bpf_dynptr_file_discard(struct bpf_dynptr *dynptr)
 {
 	struct bpf_dynptr_kern *ptr = (struct bpf_dynptr_kern *)dynptr;

kernel/bpf/verifier.c

@@ -3124,7 +3124,8 @@ struct bpf_kfunc_btf_tab {
 	u32 nr_descs;
 };
 
-static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc *desc);
+static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc *desc,
+			    int insn_idx);
 
 static int kfunc_desc_cmp_by_id_off(const void *a, const void *b)
 {
@@ -21869,7 +21870,7 @@ static int fixup_call_args(struct bpf_verifier_env *env)
 }
 
 /* replace a generic kfunc with a specialized version if necessary */
-static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc *desc)
+static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc *desc, int insn_idx)
 {
 	struct bpf_prog *prog = env->prog;
 	bool seen_direct_write;
@@ -21904,6 +21905,9 @@ static int specialize_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_desc
 	} else if (func_id == special_kfunc_list[KF_bpf_remove_dentry_xattr]) {
 		if (bpf_lsm_has_d_inode_locked(prog))
 			addr = (unsigned long)bpf_remove_dentry_xattr_locked;
+	} else if (func_id == special_kfunc_list[KF_bpf_dynptr_from_file]) {
+		if (!env->insn_aux_data[insn_idx].non_sleepable)
+			addr = (unsigned long)bpf_dynptr_from_file_sleepable;
 	}
 
 set_imm:
@@ -21963,7 +21967,7 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 		return -EFAULT;
 	}
 
-	err = specialize_kfunc(env, desc);
+	err = specialize_kfunc(env, desc, insn_idx);
 	if (err)
 		return err;