[U-Boot] [PATCH] Revert "part: Allocate only one legacy_mbr buffer"

[Alexey Brodkin]

Hi Faiz,

> -----Original Message-----
> From: Alexey Brodkin
> Sent: Wednesday, September 4, 2019 4:23 PM
> To: Faiz Abbas <faiz_abbas at ti.com>
> Cc: paulemge at forallsecure.com; trini at konsulko.com; u-boot at lists.denx.de
> Subject: RE: [PATCH] Revert "part: Allocate only one legacy_mbr buffer"
> 
> Hi Faiz,
> 
> > -----Original Message-----
> > From: Faiz Abbas <faiz_abbas at ti.com>
> > Sent: Wednesday, September 4, 2019 4:09 PM
> > To: Alexey Brodkin <abrodkin at synopsys.com>
> > Cc: paulemge at forallsecure.com; trini at konsulko.com; u-boot at lists.denx.de
> > Subject: Re: [PATCH] Revert "part: Allocate only one legacy_mbr buffer"
> >
> > Hi Alexey,
> >
> > On 04/09/19 6:27 PM, Alexey Brodkin wrote:
> > > Hi Faiz,
> > >
> > > [snip]
> > >
> > >>>>> I guess what you really want to do is to allocate buffer for "mbr"
> > >>>>> dynamically of size which is max(sizeof(legacy_mbr), dev_desc->blksz).
> > >>>>>
> > >>>>
> > >>>> With the assumption that blksz is always greater than
> > >>>> sizeof(legacy_mbr), this should work:
> > >>>>
> > >>>> ALLOC_CACHE_ALIGN_BUFFER(legacy_mbr, mbr, DIV_ROUND_UP(dev_desc->blksz,
> > >>>> sizeof(legacy_mbr)));
> > >>>
> > >>> No this won't work. See ALLOC_CACHE_ALIGN_BUFFER does static allocation
> > >>> in compile-time while blksz is set in runtime.
> > >>>
> > >>> That's why I mentioned switch to runtime allocation.
> > >>>
> > >>
> > >> Hmm. So the problem isn't as much about allocating too much in the
> > >> buffer but about using dynamically determined size for static array
> > >> declaration.
> > >
> > > In fact it was a problem of huge static allocation I discovered just by
> > > chance building U-Boot for a very memory-limited device, see [1].
> > >
> > >> This is being used all over this disk/part_dos.c file.
> > >> Shouldn't we fix all of that? Not sure how it has been working all along.
> > >
> > > That part I don't quite understand. What's being used, where and how?
> > > And what's the problem with dynamic allocation of the buffer for MBR?
> > >
> >
> > Isn't the following line (being used in different functions in
> > disk/part_dos.c) also problematic because we don't know the value of
> > blksz at compile time?
> >
> > ALLOC_CACHE_ALIGN_BUFFER(unsigned char, buffer, dev_desc->blksz);
> 
> Ok I see now. Looks like I missed that Variable-Length Array (VLA) feature
> added to C99 so up-to-date GCC apparently supports it.
> 
> But I would strongly recommend to get rid of VLA usage by all means,
> see [1] & [2].
> 
> [1] https://lwn.net/Articles/749064/
> [2] https://lwn.net/Articles/749089/

So if I add "-Wvla" to CFLAGS I see 22 usages of them:
-------------------------------->8---------------------------------
diff --git a/Makefile b/Makefile
index c02accfc26..c6e8d12809 100644
--- a/Makefile
+++ b/Makefile
@@ -389,7 +389,7 @@ CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \

 KBUILD_CPPFLAGS := -D__KERNEL__ -D__UBOOT__

-KBUILD_CFLAGS   := -Wall -Wstrict-prototypes \
+KBUILD_CFLAGS   := -Wvla -Wall -Wstrict-prototypes \
                   -Wno-format-security \
                   -fno-builtin -ffreestanding $(CSTD_FLAG)
 KBUILD_CFLAGS  += -fshort-wchar -fno-strict-aliasing
-------------------------------->8---------------------------------

So that's what we have:
-------------------------------->8---------------------------------
# make -j 48 2>&1 | grep "\-Wvla"
disk/part_dos.c:129:2: warning: ISO C90 forbids variable length array ‘__buffer’ [-Wvla]
disk/part_dos.c:200:2: warning: ISO C90 forbids variable length array ‘__buffer’ [-Wvla]
drivers/spi/spi-mem.c:350:2: warning: ISO C90 forbids variable length array ‘op_buf’ [-Wvla]
drivers/input/input.c:341:2: warning: ISO C90 forbids variable length array ‘temp’ [-Wvla]
drivers/input/input.c:513:2: warning: ISO C90 forbids variable length array ‘ch’ [-Wvla]
env/attr.c:124:2: warning: ISO C90 forbids variable length array ‘regex’ [-Wvla]
env/attr.c:129:10: warning: ISO C90 forbids variable length array ‘caps’ [-Wvla]
common/command.c:29:3: warning: ISO C90 forbids variable length array ‘cmd_array’ [-Wvla]
drivers/mmc/dw_mmc.c:248:34: warning: ISO C90 forbids variable length array ‘__cur_idmac’ [-Wvla]
fs/fat/fat.c:61:2: warning: ISO C90 forbids variable length array ‘__buffer’ [-Wvla]
fs/fat/fat.c:262:3: warning: ISO C90 forbids variable length array ‘__tmpbuf’ [-Wvla]
fs/fat/fat.c:290:3: warning: ISO C90 forbids variable length array ‘__tmpbuf’ [-Wvla]
fs/fat/fat_write.c:410:3: warning: ISO C90 forbids variable length array ‘__tmpbuf’ [-Wvla]
fs/fat/fat_write.c:439:3: warning: ISO C90 forbids variable length array ‘__tmpbuf’ [-Wvla]
fs/ext4/ext4_common.c:200:2: warning: ISO C90 forbids variable length array ‘__sec_buf’ [-Wvla]
fs/fs_internal.c:18:2: warning: ISO C90 forbids variable length array ‘__sec_buf’ [-Wvla]
fs/fs_internal.c:62:3: warning: ISO C90 forbids variable length array ‘__p’ [-Wvla]
fs/ext4/ext4_common.c:2075:4: warning: ISO C90 forbids variable length array ‘filename’ [-Wvla]
fs/ext4/ext4_common.c:2213:2: warning: ISO C90 forbids variable length array ‘fpath’ [-Wvla]
lib/fdtdec.c:395:2: warning: ISO C90 forbids variable length array ‘nodes’ [-Wvla]
lib/hashtable.c:601:9: warning: ISO C90 forbids variable length array ‘list’ [-Wvla]
lib/hashtable.c:792:2: warning: ISO C90 forbids variable length array ‘localvars’ [-Wvla]
-------------------------------->8---------------------------------

Given that situation I think it should be fine for starters to implement
a fix proposed by you and later work on VLA removal as a separate project.

-Alexey