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    SubjectRe: [PATCH AUTOSEL 4.4 080/100] char: hpet: Use flexible-array member
    On Fri, Feb 14, 2020 at 09:43:14AM -0800, Eric Biggers wrote:
    >On Fri, Feb 14, 2020 at 11:24:04AM -0500, Sasha Levin wrote:
    >> From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
    >>
    >> [ Upstream commit 987f028b8637cfa7658aa456ae73f8f21a7a7f6f ]
    >>
    >> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
    >> presence of a "variable length array":
    >>
    >> struct something {
    >> int length;
    >> u8 data[1];
    >> };
    >>
    >> struct something *instance;
    >>
    >> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
    >> instance->length = size;
    >> memcpy(instance->data, source, size);
    >>
    >> There is also 0-byte arrays. Both cases pose confusion for things like
    >> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
    >> to declare variable-length types such as the one above is a flexible array
    >> member[2] which need to be the last member of a structure and empty-sized:
    >>
    >> struct something {
    >> int stuff;
    >> u8 data[];
    >> };
    >>
    >> Also, by making use of the mechanism above, we will get a compiler warning
    >> in case the flexible array does not occur last in the structure, which
    >> will help us prevent some kind of undefined behavior bugs from being
    >> unadvertenly introduced[3] to the codebase from now on.
    >>
    >> [1] https://github.com/KSPP/linux/issues/21
    >> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
    >> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
    >>
    >> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
    >> Link: https://lore.kernel.org/r/20200120235326.GA29231@embeddedor.com
    >> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    >> Signed-off-by: Sasha Levin <sashal@kernel.org>
    >> ---
    >> drivers/char/hpet.c | 2 +-
    >> 1 file changed, 1 insertion(+), 1 deletion(-)
    >>
    >> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
    >> index 5b38d7a8202a1..38c2ae93ce492 100644
    >> --- a/drivers/char/hpet.c
    >> +++ b/drivers/char/hpet.c
    >> @@ -112,7 +112,7 @@ struct hpets {
    >> unsigned long hp_delta;
    >> unsigned int hp_ntimer;
    >> unsigned int hp_which;
    >> - struct hpet_dev hp_dev[1];
    >> + struct hpet_dev hp_dev[];
    >> };
    >>
    >
    >Umm, why are you backporting this without the commit that fixes it? Does your

    mhm, for some reason it failed to apply to 4.19 and older. I can look at
    that.

    >AUTOSEL process really still not pay attention to Fixes tags? They are there
    >for a reason.

    Yes, it looks at the Fixes tag, thank you for the explanation.

    >And for that matter, why are you backporting it all, given that this is a
    >cleanup and not a fix?

    If I recall correctly CONFIG_FORTIFY_SOURCE=y results in user visible
    warnings, which we try to fix in the stable kernel.

    --
    Thanks,
    Sasha

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