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Date: Wed, 9 Feb 2022 19:04:07 -0600 From: "Gustavo A. R. Silva" <gustavoars@...nel.org> To: Kees Cook <keescook@...omium.org> Cc: Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Keith Busch <kbusch@...nel.org>, linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org Subject: Re: [PATCH] overflow: Provide constant expression struct_size On Wed, Feb 09, 2022 at 04:43:26PM -0800, Kees Cook wrote: > There have been cases where struct_size() (or flex_array_size()) needs > to be calculated for an initializer, which requires it be a constant > expression. This is possible when the "count" argument is a constant > expression, so provide this ability for the helpers. > > Signed-off-by: Kees Cook <keescook@...omium.org> Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org> Tested-by: Gustavo A. R. Silva <gustavoars@...nel.org> Thanks! -- Gustavo > --- > include/linux/overflow.h | 10 +++++++--- > lib/test_overflow.c | 7 +++++++ > 2 files changed, 14 insertions(+), 3 deletions(-) > > diff --git a/include/linux/overflow.h b/include/linux/overflow.h > index 59d7228104d0..f1221d11f8e5 100644 > --- a/include/linux/overflow.h > +++ b/include/linux/overflow.h > @@ -4,6 +4,7 @@ > > #include <linux/compiler.h> > #include <linux/limits.h> > +#include <linux/const.h> > > /* > * We need to compute the minimum and maximum values representable in a given > @@ -221,8 +222,9 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) > * Return: number of bytes needed or SIZE_MAX on overflow. > */ > #define flex_array_size(p, member, count) \ > - size_mul(count, \ > - sizeof(*(p)->member) + __must_be_array((p)->member)) > + __builtin_choose_expr(__is_constexpr(count), \ > + (count) * sizeof(*(p)->member) + __must_be_array((p)->member), \ > + size_mul(count, sizeof(*(p)->member) + __must_be_array((p)->member))) > > /** > * struct_size() - Calculate size of structure with trailing flexible array. > @@ -237,6 +239,8 @@ static inline size_t __must_check size_sub(size_t minuend, size_t subtrahend) > * Return: number of bytes needed or SIZE_MAX on overflow. > */ > #define struct_size(p, member, count) \ > - size_add(sizeof(*(p)), flex_array_size(p, member, count)) > + __builtin_choose_expr(__is_constexpr(count), \ > + sizeof(*(p)) + flex_array_size(p, member, count), \ > + size_add(sizeof(*(p)), flex_array_size(p, member, count))) > > #endif /* __LINUX_OVERFLOW_H */ > diff --git a/lib/test_overflow.c b/lib/test_overflow.c > index 712fb2351c27..d488ffaf56be 100644 > --- a/lib/test_overflow.c > +++ b/lib/test_overflow.c > @@ -602,11 +602,18 @@ struct __test_flex_array { > > static int __init test_overflow_size_helpers(void) > { > + /* Make sure struct_size() can be used in a constant expression. */ > + u8 ce_array[struct_size((struct __test_flex_array *)0, data, 55)]; > struct __test_flex_array *obj; > int count = 0; > int err = 0; > int var; > > + /* Verify constant expression against runtime version. */ > + var = 55; > + OPTIMIZER_HIDE_VAR(var); > + err |= sizeof(ce_array) != struct_size(obj, data, var); > + > #define check_one_size_helper(expected, func, args...) ({ \ > bool __failure = false; \ > size_t _r; \ > -- > 2.30.2 >
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