Merge branch 'allow-variable-offsets-for-syscall-ptr_to_ctx'

Merge branch 'allow-variable-offsets-for-syscall-ptr_to_ctx'

Kumar Kartikeya Dwivedi says:

====================
Allow variable offsets for syscall PTR_TO_CTX

Enable pointer modification with variable offsets accumulated in the
register for PTR_TO_CTX for syscall programs where it won't be
rewritten, and the context is user-supplied and checked against the max
offset. See patches for details. Fixed offset support landed in [0].

By combining this set with [0], examples like the one below should
succeed verification now.

  SEC("syscall")
  int prog(void *ctx) {
	int *arr = ctx;
	int i;

	bpf_for(i, 0, 100)
		arr[i] *= i;

	return 0;
  }

  [0]: https://lore.kernel.org/bpf/20260227005725.1247305-1-memxor@gmail.com

Changelog:
----------
v4 -> v5
v4: https://lore.kernel.org/bpf/20260401122818.2240807-1-memxor@gmail.com

 * Use is_var_ctx_off_allowed() consistently.
 * Add acks. (Emil)

v3 -> v4
v3: https://lore.kernel.org/bpf/20260318103526.2590079-1-memxor@gmail.com

 * Drop comment around describing choice of fixed or variable offsets. (Eduard)
 * Simplify offset adjustment for different cases. (Eduard)
 * Add PTR_TO_CTX case in __check_mem_access(). (Eduard)
 * Drop aligned access constraint from syscall_prog_is_valid_access().
 * Wrap naked checks for BPF_PROG_TYPE_SYSCALL in a utility function. (Eduard)
 * Split tests into separate clean up and addition patches. (Eduard)
 * Remove CAP_SYS_ADMIN changes. (Eduard)
 * Enable unaligned access to syscall ctx, add tests.
 * Add more tests for various corner cases.
 * Add acks. (Puranjay, Mykyta)

v2 -> v3
v2: https://lore.kernel.org/bpf/20260318075133.1031781-1-memxor@gmail.com

 * Prevent arg_type for KF_ARG_PTR_TO_CTX from applying to other cases
   due to preceding fallthrough. (Gemini/Sashiko)

v1 -> v2
v1: https://lore.kernel.org/bpf/20260317111850.2107846-2-memxor@gmail.com

 * Harden check_func_arg_reg_off check with ARG_PTR_TO_CTX.
 * Add tests for unmodified ctx into tail calls.
 * Squash unmodified ctx change into base commit.
 * Add Reviewed-by's from Emil.
====================

Link: https://patch.msgid.link/20260406194403.1649608-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>