bpf: Add bpf_copy_from_user_task() helper

This adds a helper for bpf programs to read the memory of other
tasks.

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 <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

include/linux/bpf.h

@@ -2243,6 +2243,7 @@ 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);

include/uapi/linux/bpf.h

@@ -5076,6 +5076,16 @@ union bpf_attr {
  * associated to *xdp_md*, at *offset*.
  * 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.
+ * Return
+ * 0 on success, or a negative error in case of failure. On error
+ * *dst* buffer is zeroed out.
  */
 #define __BPF_FUNC_MAPPER(FN) \
  FN(unspec), \
@@ -5269,6 +5279,7 @@ union bpf_attr {
  FN(xdp_get_buff_len), \
  FN(xdp_load_bytes), \
  FN(xdp_store_bytes), \
+ FN(copy_from_user_task), \
  /* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper

kernel/bpf/helpers.c

@@ -16,6 +16,7 @@
 #include <linux/pid_namespace.h>
 #include <linux/proc_ns.h>
 #include <linux/security.h>
+#include <linux/btf_ids.h>
 
 #include "../../lib/kstrtox.h"
 
@@ -671,6 +672,39 @@ const struct bpf_func_proto bpf_copy_from_user_proto = {
  .arg3_type = ARG_ANYTHING,
 };
 
+BPF_CALL_5(bpf_copy_from_user_task, void *, dst, u32, size,
+ const void __user *, user_ptr, struct task_struct *, tsk, u64, flags)
+{
+ int ret;
+
+ /* flags is not used yet */
+ if (unlikely(flags))
+ return -EINVAL;
+
+ if (unlikely(!size))
+ return 0;
+
+ ret = access_process_vm(tsk, (unsigned long)user_ptr, dst, size, 0);
+ if (ret == size)
+ return 0;
+
+ memset(dst, 0, size);
+ /* Return -EFAULT for partial read */
+ return ret < 0 ? ret : -EFAULT;
+}
+
+const struct bpf_func_proto bpf_copy_from_user_task_proto = {
+ .func = bpf_copy_from_user_task,
+ .gpl_only = false,
+ .ret_type = RET_INTEGER,
+ .arg1_type = ARG_PTR_TO_UNINIT_MEM,
+ .arg2_type = ARG_CONST_SIZE_OR_ZERO,
+ .arg3_type = ARG_ANYTHING,
+ .arg4_type = ARG_PTR_TO_BTF_ID,
+ .arg4_btf_id = &btf_tracing_ids[BTF_TRACING_TYPE_TASK],
+ .arg5_type = ARG_ANYTHING
+};
+
 BPF_CALL_2(bpf_per_cpu_ptr, const void *, ptr, u32, cpu)
 {
  if (cpu >= nr_cpu_ids)

kernel/trace/bpf_trace.c

@@ -1235,6 +1235,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
  return &bpf_get_task_stack_proto;
  case BPF_FUNC_copy_from_user:
  return prog->aux->sleepable ? &bpf_copy_from_user_proto : NULL;
+ case BPF_FUNC_copy_from_user_task:
+ return prog->aux->sleepable ? &bpf_copy_from_user_task_proto : NULL;
  case BPF_FUNC_snprintf_btf:
  return &bpf_snprintf_btf_proto;
  case BPF_FUNC_per_cpu_ptr:

tools/include/uapi/linux/bpf.h

@@ -5076,6 +5076,16 @@ union bpf_attr {
  * associated to *xdp_md*, at *offset*.
  * 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.
+ * Return
+ * 0 on success, or a negative error in case of failure. On error
+ * *dst* buffer is zeroed out.
  */
 #define __BPF_FUNC_MAPPER(FN) \
  FN(unspec), \
@@ -5269,6 +5279,7 @@ union bpf_attr {
  FN(xdp_get_buff_len), \
  FN(xdp_load_bytes), \
  FN(xdp_store_bytes), \
+ FN(copy_from_user_task), \
  /* */
 
 /* integer value in 'imm' field of BPF_CALL instruction selects which helper