[android-common:upstream-f2fs-stable-linux-4.19.y 289/1172] fs/crypto/hkdf.c:47:9: sparse: sparse: Variable length array is used.

[kernel test robot <lkp@intel.com>]

tree:   https://android.googlesource.com/kernel/common upstream-f2fs-stable-linux-4.19.y
head:   b9aeb147225616494256fcf913c559afd4088a05
commit: 6ad6af5912f72ad8c40baa072c3dabd321695dc1 [289/1172] fscrypt: add an HKDF-SHA512 implementation
config: i386-randconfig-s052-20230614 (https://download.01.org/0day-ci/archive/20230614/202306141910.WL32Ks9Y-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.4-39-gce1a6720-dirty
        git remote add android-common https://android.googlesource.com/kernel/common
        git fetch --no-tags android-common upstream-f2fs-stable-linux-4.19.y
        git checkout 6ad6af5912f72ad8c40baa072c3dabd321695dc1
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' O=build_dir ARCH=i386 SHELL=/bin/bash drivers/cpuidle/ drivers/of/ fs/crypto/ kernel/dma/

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202306141910.WL32Ks9Y-lkp@intel.com/

sparse warnings: (new ones prefixed by >>)
>> fs/crypto/hkdf.c:47:9: sparse: sparse: Variable length array is used.
   fs/crypto/hkdf.c:120:9: sparse: sparse: Variable length array is used.
   In file included from include/linux/gfp.h:6,
                    from include/linux/slab.h:15,
                    from include/linux/crypto.h:24,
                    from include/crypto/hash.h:16,
                    from fs/crypto/hkdf.c:12:
   include/linux/mmzone.h: In function '__nr_to_section':
   include/linux/mmzone.h:1156:13: warning: the comparison will always evaluate as 'true' for the address of 'mem_section' will never be NULL [-Waddress]
    1156 |         if (!mem_section[SECTION_NR_TO_ROOT(nr)])
         |             ^
   include/linux/mmzone.h:1147:27: note: 'mem_section' declared here
    1147 | extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
         |                           ^~~~~~~~~~~

vim +47 fs/crypto/hkdf.c

    25	
    26	/*
    27	 * HKDF consists of two steps:
    28	 *
    29	 * 1. HKDF-Extract: extract a pseudorandom key of length HKDF_HASHLEN bytes from
    30	 *    the input keying material and optional salt.
    31	 * 2. HKDF-Expand: expand the pseudorandom key into output keying material of
    32	 *    any length, parameterized by an application-specific info string.
    33	 *
    34	 * HKDF-Extract can be skipped if the input is already a pseudorandom key of
    35	 * length HKDF_HASHLEN bytes.  However, cipher modes other than AES-256-XTS take
    36	 * shorter keys, and we don't want to force users of those modes to provide
    37	 * unnecessarily long master keys.  Thus fscrypt still does HKDF-Extract.  No
    38	 * salt is used, since fscrypt master keys should already be pseudorandom and
    39	 * there's no way to persist a random salt per master key from kernel mode.
    40	 */
    41	
    42	/* HKDF-Extract (RFC 5869 section 2.2), unsalted */
    43	static int hkdf_extract(struct crypto_shash *hmac_tfm, const u8 *ikm,
    44				unsigned int ikmlen, u8 prk[HKDF_HASHLEN])
    45	{
    46		static const u8 default_salt[HKDF_HASHLEN];
  > 47		SHASH_DESC_ON_STACK(desc, hmac_tfm);
    48		int err;
    49	
    50		err = crypto_shash_setkey(hmac_tfm, default_salt, HKDF_HASHLEN);
    51		if (err)
    52			return err;
    53	
    54		desc->tfm = hmac_tfm;
    55		desc->flags = 0;
    56		err = crypto_shash_digest(desc, ikm, ikmlen, prk);
    57		shash_desc_zero(desc);
    58		return err;
    59	}
    60	

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