Re: [PATCH AUTOSEL 4.4 080/100] char: hpet: Use flexible-array member

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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@xxxxxxxxxxxxxx>

[ 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@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20200120235326.GA29231@xxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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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