Re: fs/ntfscomp.c:82:11: error: ‘flg’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

[Paul Menzel]

Dear PGNet Dev,


Thank you for your report.


Am 26.03.20 um 04:50 schrieb PGNet Dev:
> building
>
>         cd grub
>         git log | head -n5
>                 commit 552c9fd08122a3036c724ce96dfe68aa2f75705f
>                 Author: Patrick Steinhardt <address@hidden>
>                 Date:   Sat Mar 7 17:29:09 2020 +0100
>
>                     gnulib: Fix build of base64 when compiling with memory 
> debugging
>
> with
>
>         gcc --version
>                 gcc (SUSE Linux) 10.0.1 20200324 (experimental) [revision 
> 75c24a08d697d6442fe6c26142f0559f803af977]
>
> patched
>
>         patch -p1 < 
> /tmp/grub_patches/grub_patches/0001-mdraid1x_linux-Fix-gcc10-error-Werror-array-bounds.patch
>                 patching file grub-core/disk/mdraid1x_linux.c
>         patch -p1 < 
> /tmp/grub_patches/grub_patches/0002-zfs-Fix-gcc10-error-Werror-zero-length-bounds.patch
>                 patching file grub-core/fs/zfs/zfs.c
>                 patching file include/grub/zfs/zap_leaf.h
>
> from
>
>         https://lists.gnu.org/archive/html/grub-devel/2020-03/msg00222.html
>
> config's ok
>
>         unset CC CPP
>         ./bootstrap
>         ./autogen.sh
>         ./configure
>
> build FAILs
>
>         make V=1
>
>                 gcc -DHAVE_CONFIG_H -I.  -Wall -W -DGRUB_UTIL=1 -D_FILE_OFFSET_BITS=64 
> -I./include -DGRUB_FILE=\"grub-core/fs/ntfscomp.c\" -I. -I. -I. -I. -I./include 
> -I./include -I./grub-core/lib/libgcrypt-grub/src/  -I./grub-core/lib/minilzo 
> -I./grub-core/lib/xzembed -I./grub-core/lib/zstd -DMINILZO_HAVE_CONFIG_H -O3 -Wall 
> -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fmessage-length=0 
> -grecord-gcc-switches -march=native -mtune=native -D_FORTIFY_SOURCE=2 
> -D_FILE_OFFSET_BITS=64 -Wall -W -Wshadow -Wpointer-arith -Wundef -Wchar-subscripts 
> -Wcomment -Wdeprecated-declarations -Wdisabled-optimization -Wdiv-by-zero -Wfloat-equal 
> -Wformat-extra-args -Wformat-security -Wformat-y2k -Wimplicit 
> -Wimplicit-function-declaration -Wimplicit-int -Wmain -Wmissing-braces 
> -Wmissing-format-attribute -Wmultichar -Wparentheses -Wreturn-type -Wsequence-point 
> -Wshadow -Wsign-compare -Wswitch -Wtrigraphs -Wunknown-pragmas -Wunused -Wunused-function 
> -Wunused-label -Wunused-parameter -Wunused-value  -Wunused-variable -Wwrite-strings 
> -Wnested-externs -Wstrict-prototypes -Wcast-align  -Wextra -Wattributes -Wendif-labels 
> -Winit-self -Wint-to-pointer-cast -Winvalid-pch -Wmissing-field-initializers -Wnonnull 
> -Woverflow -Wvla -Wpointer-to-int-cast -Wstrict-aliasing -Wvariadic-macros 
> -Wvolatile-register-var -Wpointer-sign -Wmissing-include-dirs -Wmissing-prototypes 
> -Wmissing-declarations -Wformat=2 -Werror  -fno-builtin -Wno-undef -O3 -Wall 
> -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fmessage-length=0 
> -grecord-gcc-switches -march=native -mtune=native -MT 
> grub-core/fs/libgrubmods_a-ntfscomp.o -MD -MP -MF 
> grub-core/fs/.deps-util/libgrubmods_a-ntfscomp.Tpo -c -o 
> grub-core/fs/libgrubmods_a-ntfscomp.o `test -f 'grub-core/fs/ntfscomp.c' || echo 
> './'`grub-core/fs/ntfscomp.c
>                 grub-core/fs/ntfscomp.c: In function ‘read_block’:
>                 grub-core/fs/ntfscomp.c:82:11: error: ‘flg’ may be used 
> uninitialized in this function [-Werror=maybe-uninitialized]
>                    82 |       if (flg & 0x8000)
>                       |           ^~~
>                 grub-core/fs/ntfscomp.c:74:17: note: ‘flg’ was declared here
>                    74 |   grub_uint16_t flg, cnt;
>                       |                 ^~~
>                 cc1: all warnings being treated as errors
>                 make[2]: *** [Makefile:7647: 
> grub-core/fs/libgrubmods_a-ntfscomp.o] Error 1
>                 make[2]: Leaving directory '/usr/local/src/grub'
>                 make[1]: *** [Makefile:11920: all-recursive] Error 1
>                 make[1]: Leaving directory '/usr/local/src/grub'
>                 make: *** [Makefile:3772: all] Error 2

I am unable to reproduce this with

    $ gcc --version
    gcc (Debian 10-20200324-1) 10.0.1 20200324 (experimental) [master 
revision 596c90d3559:023579257f5:906b3eb9df6c577d3f6e9c3ea5c9d7e4d1e90536]

Here is the code in question:

``
static grub_err_t
decomp_get16 (struct grub_ntfs_comp *cc, grub_uint16_t * res)
{
  grub_uint8_t c1 = 0, c2 = 0;

  if ((decomp_getch (cc, &c1)) || (decomp_getch (cc, &c2)))
    return grub_errno;
  *res = ((grub_uint16_t) c2) * 256 + ((grub_uint16_t) c1);
  return 0;
}

[…]
  grub_uint16_t flg, cnt;

  if (decomp_get16 (cc, &flg))
    return grub_errno;
  cnt = (flg & 0xFFF) + 1;

  if (dest)
    {
      if (flg & 0x8000)
[…]
```

In the code, the `if (decomp_get16 (cc, &flg))` ensures, that `flg` is 
initialized, doesn’t it?

Why is it complaining complaining in line 82 and not 78, where `flg` is 
already accessed?


Kind regards,

Paul