Merge branch 'bpf-additional-use-cases-for-untrusted-ptr_to_mem'

Eduard Zingerman says:

====================
bpf: additional use-cases for untrusted PTR_TO_MEM

This patch set introduces two usability enhancements leveraging
untrusted pointers to mem:
- When reading a pointer field from a PTR_TO_BTF_ID, the resulting
  value is now assumed to be PTR_TO_MEM|MEM_RDONLY|PTR_UNTRUSTED
  instead of SCALAR_VALUE, provided the pointer points to a primitive
  type.
- __arg_untrusted attribute for global function parameters,
  allowed for pointer arguments of both structural and primitive
  types:
  - For structural types, the attribute produces
    PTR_TO_BTF_ID|PTR_UNTRUSTED.
  - For primitive types, it yields
    PTR_TO_MEM|MEM_RDONLY|PTR_UNTRUSTED.

Here are examples enabled by the series:

  struct foo {
    int *arr;
  };
  ...
  p = bpf_core_cast(..., struct foo);
  bpf_for(i, 0, ...) {
    ... p->arr[i] ...       // load at any offset is allowed
  }

  int memcmp(void *a __arg_untrusted, void *b __arg_untrusted, size_t n) {
    bpf_for(i, 0, n)
      if (a[i] - b[i])      // load at any offset is allowed
        return ...;
    return 0;
  }

The patch-set was inspired by Anrii's series [1]. The goal of that
series was to define a generic global glob_match function, capable to
accept any pointer type:

  __weak int glob_match(const char *pat, const char *str);

  char filename_glob[32];

  void foo(...) {
    ...
    task = bpf_get_current_task_btf();
    filename = task->mm->exe_file->f_path.dentry->d_name.name;
    ... match_glob(filename_glob,  // pointer to map value
                   filename) ...   // scalar
  }

At the moment, there is no straightforward way to express such a
function. This patch-set makes it possible to define it as follows:

  __weak int glob_match(const char *pat __arg_untrusted,
                        const char *str __arg_untrusted);

[1] https://github.com/anakryiko/linux/tree/bpf-mem-cast

Changelog:
v1: https://lore.kernel.org/bpf/20250702224209.3300396-1-eddyz87@gmail.com/
v1 -> v2:
- Added safety check in btf_prepare_func_args() to ensure that only
  struct or primitive types could be combined with __arg_untrusted
  (Alexei).
- Removed unnecessary 'continue' btf_check_func_arg_match() (Alexei).
- Added test cases for __arg_untrusted pointers to enum and
  __arg_untrusted combined with non-kernel type (Kumar).
- Added acks from Kumar.
====================

Link: https://patch.msgid.link/20250704230354.1323244-1-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>