Modern (fortified) memcpy() prefers to avoid writing (or reading) beyond
the end of the addressed destination (or source) struct member:

In function ‘fortify_memcpy_chk’,
    inlined from ‘syscall_get_arguments’ at ./arch/x86/include/asm/syscall.h:85:2,
    inlined from ‘populate_seccomp_data’ at kernel/seccomp.c:258:2,
    inlined from ‘__seccomp_filter’ at kernel/seccomp.c:1231:3:
./include/linux/fortify-string.h:580:25: error: call to ‘__read_overflow2_field’ declared with attribute warning: detected read beyond size of field (2nd parameter); maybe use struct_group()? [-Werror=attribute-warning]
  580 |                         __read_overflow2_field(q_size_field, size);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As already done for x86_64 and compat mode, do not use memcpy() to
extract syscall arguments from struct pt_regs but rather just perform
direct assignments. Binary output differences are negligible, and actually
ends up using less stack space:

-       sub    $0x84,%esp
+       sub    $0x6c,%esp

and less text size:

   text    data     bss     dec     hex filename
  10794     252       0   11046    2b26 gcc-32b/kernel/seccomp.o.stock
  10714     252       0   10966    2ad6 gcc-32b/kernel/seccomp.o.after

Closes: https://lore.kernel.org/lkml/9b69fb14-df89-4677-9c82-056ea9e706f5@gmail.com/Reported-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Tested-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Link: https://lore.kernel.org/all/20240708202202.work.477-kees%40kernel.org