Re: possible array out of bounds access in sys/netinet/sctp_output.c

On Nov 27, 2011, at 5:24 PM, Jilles Tjoelker wrote:

> On Sun, Nov 27, 2011 at 03:45:36PM +0000, Alexander Best wrote:
>> i've been playing with clang tot and noticed the following error:
> 
>> /usr/local/bin/clang -c -O3 -pipe -fno-inline-functions -fno-strict-aliasing -march=core2 -std=c99 -g -fdiagnostics-show-option -fformat-extensions -Wall  -Wcast-qual -Winline -Wmissing-include-dirs  -Wmissing-prototypes -Wnested-externs -Wpointer-arith  -Wredundant-decls -Wstrict-prototypes -Wundef  -Wno-pointer-sign -nostdinc  -I. -I/usr/git-freebsd-head/sys -I/usr/git-freebsd-head/sys/contrib/altq -D_KERNEL -DHAVE_KERNEL_OPTION_HEADERS -include opt_global.h  -fno-omit-frame-pointer -mno-aes -mno-avx -mcmodel=kernel -mno-red-zone -mno-mmx -msoft-float  -fno-asynchronous-unwind-tables -ffreestanding -Wno-error=tautological-compare -Wno-error=shift-count-negative  -Wno-error=shift-count-overflow -Wno-error=shift-overflow -Wno-error=conversion  -Wno-error=empty-body -Wno-error=gnu-designator -Wno-error=format  -Wno-error=format-invalid-specifier -Wno-error=format-extra-args -Werror  /usr/git-freebsd-head/sys/netinet/sctp_output.c
>> clang: warning: argument unused during compilation: '-fformat-extensions'
>> /usr/git-freebsd-head/sys/netinet/sctp_output.c:4685:2: error: array index 1 is past the end of the array (which contains 1 element) [-Werror,-Warray-bounds]
>>        sup_addr->addr_type[1] = htons(SCTP_IPV6_ADDRESS);
>>        ^                   ~
>> /usr/git-freebsd-head/sys/netinet/sctp_header.h:84:2: note: array 'addr_type' declared here
>>        uint16_t addr_type[SCTP_ARRAY_MIN_LEN]; /* array of supported address
>>        ^
>> 1 error generated.
>> *** Error code 1
>> 
>> Stop in /usr/obj/usr/git-freebsd-head/sys/GENERIC.
>> *** Error code 1
>> 
>> Stop in /usr/git-freebsd-head.
>> *** Error code 1
>> 
>> Stop in /usr/git-freebsd-head.
> 
>> this is from a GENERIC kernel build (so INET + INET6) for amd64. is this a
>> false positive, or is length(sup_addr->addr_type) really == 1, thus making
>> sup_addr->addr_type[1] an illegal access?
> 
> This is the fairly common construct of a variable-length array at the
> end of a struct. With C89, this was not allowed but defining one element
> and allocating more elements worked in most implementations. C99
> recognized this need and created a way to do it, which looks like
> uint16_t addr_type[];. This adds any necessary padding and allows access
> to however many elements have been allocated. Also, if it is not at the
> end of a struct it is an error.
> 
> Using this new construct requires code changes because some code such as
> fairly close to the error message relies on the size of the one element
> already in the struct.
Hi Jilles,

you are completely right. It is a false positive.

the reason why we don't use addr_type[] is that the same code is used
on different plattforms and (at least at one point of time), using
addr_type[] didn't work there.


However, reconsidering the code right now, I guess one could change to code
in a way to avoid the warning. I'll put that on my ToDo list. But it is only
to avoid the warning, there is no real problem as said earlier.

Best regards
Michael
> 
> -- 
> Jilles Tjoelker
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