Subject: Re: [PATCH v7 bpf-next 09/14] bpd: add multi-buffer support to xdp copy helpers

From: kernel test robot <[email-masked]>
Date: Sat, 20 Mar 2021 08:51:23 +0800

Hi Lorenzo,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on bpf-next/master]

url:    https://github.com/0day-ci/linux/commits/Lorenzo-Bianconi/mvneta-introduce-XDP-multi-buffer-support/20210320-055103
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: arm-randconfig-r023-20210318 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 436c6c9c20cc522c92a923440a5fc509c342a7db)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install arm cross compiling tool for clang build
        # apt-get install binutils-arm-linux-gnueabi
        # https://github.com/0day-ci/linux/commit/9603affe766576f892f3a8e02d58dbe83092c74a
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Lorenzo-Bianconi/mvneta-introduce-XDP-multi-buffer-support/20210320-055103
        git checkout 9603affe766576f892f3a8e02d58dbe83092c74a
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=arm 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <[email-masked]>

All warnings (new ones prefixed by >>):

>> net/core/filter.c:4579:16: warning: variable 'copy_len' is used uninitialized whenever 'for' loop exits because its condition is false [-Wsometimes-uninitialized]
                           for (i = 0; i < xdp_sinfo->nr_frags; i++) {
                                       ^~~~~~~~~~~~~~~~~~~~~~~
   net/core/filter.c:4593:30: note: uninitialized use occurs here
                   memcpy(dst_buff, src_buff, copy_len);
                                              ^~~~~~~~
   net/core/filter.c:4579:16: note: remove the condition if it is always true
                           for (i = 0; i < xdp_sinfo->nr_frags; i++) {
                                       ^~~~~~~~~~~~~~~~~~~~~~~
   net/core/filter.c:4570:25: note: initialize the variable 'copy_len' to silence this warning
                   unsigned long copy_len;
                                         ^
                                          = 0
   1 warning generated.

vim +4579 net/core/filter.c

  4551	
  4552	static unsigned long bpf_xdp_copy(void *dst_buff, const void *ctx,
  4553					  unsigned long off, unsigned long len)
  4554	{
  4555		struct xdp_buff *xdp = (struct xdp_buff *)ctx;
  4556		struct xdp_shared_info *xdp_sinfo;
  4557		unsigned long base_len;
  4558		const void *src_buff;
  4559	
  4560		if (likely(!xdp->mb)) {
  4561			src_buff = xdp->data;
  4562			memcpy(dst_buff, src_buff + off, len);
  4563	
  4564			return 0;
  4565		}
  4566	
  4567		base_len = xdp->data_end - xdp->data;
  4568		xdp_sinfo = xdp_get_shared_info_from_buff(xdp);
  4569		do {
  4570			unsigned long copy_len;
  4571	
  4572			if (off < base_len) {
  4573				src_buff = xdp->data + off;
  4574				copy_len = min(len, base_len - off);
  4575			} else {
  4576				unsigned long frag_off_total = base_len;
  4577				int i;
  4578	
> 4579				for (i = 0; i < xdp_sinfo->nr_frags; i++) {
  4580					skb_frag_t *frag = &xdp_sinfo->frags[i];
  4581					unsigned long frag_len = xdp_get_frag_size(frag);
  4582					unsigned long frag_off = off - frag_off_total;
  4583	
  4584					if (frag_off < frag_len) {
  4585						src_buff = xdp_get_frag_address(frag) +
  4586							   frag_off;
  4587						copy_len = min(len, frag_len - frag_off);
  4588						break;
  4589					}
  4590					frag_off_total += frag_len;
  4591				}
  4592			}
  4593			memcpy(dst_buff, src_buff, copy_len);
  4594			off += copy_len;
  4595			len -= copy_len;
  4596			dst_buff += copy_len;
  4597		} while (len);
  4598	
  4599		return 0;
  4600	}
  4601	

---
0-DAY CI Kernel Test Service, Intel Corporation
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