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

Jilles Tjoelker jilles at stack.nl
Sun Nov 27 16:24:32 UTC 2011

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.

Jilles Tjoelker

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