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

[Michael Chang]

On Thu, Mar 26, 2020 at 09:39:16AM +0100, Paul Menzel wrote:
> 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]

Looking into build log, the build option seems to have been overridden
with CFLAGS settings like this.

CFLAGS="-O3 -Wall -fstack-protector-strong -funwind-tables
-fasynchronous-unwind-tables -fmessage-length=0 -grecord-gcc-switches
-march=native -mtune=native"

I'm not sure if -O3 is considered as supported  since that will result
in larger binaries we are striving to reduce all the time. Also the
optimization it brings would require careful review if we don't enable
it by default.

In addition, -fstack-protector-strong breaks the build even harder with
a lot of __stack_chk_fail undefined symbol in the modules.

If going with default build option, I also don't have this compliation
error.

> 
> 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?

The code also looks sanity to me. It certainly has things to do with the
-O3 optimization, but I couldn't tell more beyond that fact either. :(

Regards,
Michael

> 
> 
> Kind regards,
> 
> Paul
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