Re: [Qemu-devel] [PATCH 4/8] usb: fix unbounded stack for xhci_dma_write_u32s

[Paolo Bonzini]

On 09/03/2016 09:07, Peter Xu wrote:
>>> > > pxdev:bin# gcc -v
>>> > > Using built-in specs.
>>> > > COLLECT_GCC=/bin/gcc
>>> > > COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-redhat-linux/4.8.5/lto-wrapper
>>> > > Target: x86_64-redhat-linux
>>> > > Configured with: ../configure --prefix=/usr --mandir=/usr/share/man 
>>> > > --infodir=/usr/share/info 
>>> > > --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap 
>>> > > --enable-shared --enable-threads=posix --enable-checking=release 
>>> > > --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions 
>>> > > --enable-gnu-unique-object --enable-linker-build-id 
>>> > > --with-linker-hash-style=gnu 
>>> > > --enable-languages=c,c++,objc,obj-c++,java,fortran,ada,go,lto 
>>> > > --enable-plugin --enable-initfini-array --disable-libgcj 
>>> > > --with-isl=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/isl-install
>>> > >  
>>> > > --with-cloog=/builddir/build/BUILD/gcc-4.8.5-20150702/obj-x86_64-redhat-linux/cloog-install
>>> > >  --enable-gnu-indirect-function --with-tune=generic 
>>> > > --with-arch_32=x86-64 --build=x86_64-redhat-linux
>>> > > Thread model: posix
>>> > > gcc version 4.8.5 20150623 (Red Hat 4.8.5-4) (GCC)
>>> > > 
>>> > > Do you know why "might not be inlinable"? Failed to figure it out
>>> > > myself as mentioned in cover letter..
>> > 
>> > It's just a difference in compiler versions.  But ARRAY_SIZE should be
>> > enough to fix it.
> It's dynamically allocated in stack, can we still use ARRAY_SIZE in
> this case?
> 
> Maybe for this case, best to use both stack and heap? malloc only if
> buffer big enough.

If you look at users, they only write about 20 bytes at most.  My
suggestion is to use your patch, and replace

    assert(__BUF_SIZE >= n);

with

    assert(n < ARRAY_SIZE(tmp));

Then you don't need the #define.

Paolo