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    SubjectRe: [PATCH] ubsan: Require GCC-8+ or Clang to use UBSAN
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    On 1/14/21 1:59 PM, Peter Zijlstra wrote:
    > On Mon, Jan 04, 2021 at 04:13:17PM +0100, Peter Zijlstra wrote:
    >> On Tue, Dec 22, 2020 at 11:04:54PM -0600, Josh Poimboeuf wrote:
    >>> GCC 7 has a known bug where UBSAN ignores '-fwrapv' and generates false
    >>> signed-overflow-UB warnings. The type mismatch between 'i' and
    >>> 'nr_segs' in copy_compat_iovec_from_user() is causing such a warning,
    >>> which also happens to violate uaccess rules:
    >>>
    >>> lib/iov_iter.o: warning: objtool: iovec_from_user()+0x22d: call to __ubsan_handle_add_overflow() with UACCESS enabled
    >>>
    >>> Fix it by making the variable types match.
    >>>
    >>> This is similar to a previous commit:
    >>>
    >>> 29da93fea3ea ("mm/uaccess: Use 'unsigned long' to placate UBSAN warnings on older GCC versions")
    >>
    >> Maybe it's time we make UBSAN builds depend on GCC-8+ ?
    >
    > ---
    > Subject: ubsan: Require GCC-8+ or Clang to use UBSAN
    >
    > Just like how we require GCC-8.2 for KASAN due to compiler bugs, require
    > a sane version of GCC for UBSAN.
    >
    > Specifically, before GCC-8 UBSAN doesn't respect -fwrapv and thinks
    > signed arithmetic is buggered.
    >

    Actually removing CONFIG_UBSAN_SIGNED_OVERFLOW would give us the same effect without restricting GCC versions.

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