Re: [PATCH] libfs: fix error format in simple_attr_write()

[Andrew Morton]

On Thu, 15 Sep 2022 09:15:44 +0000 Eliav Farber <farbere@xxxxxxxxxx> wrote:

> In commit 488dac0c9237 ("libfs: fix error cast of negative value in
> simple_attr_write()"), simple_attr_write() was changed to use kstrtoull()
> instead of simple_strtoll() to convert a string got from a user.
> A user trying to set a negative value will get an error.
> 
> This is wrong since it breaks all the places that use
> DEFINE_DEBUGFS_ATTRIBUTE() with format of a signed integer.
> 
> For the record there are 43 current users of signed integer which are
> likely to be effected by this:
> 
> $ git grep -n -A1 -w DEFINE_DEBUGFS_ATTRIBUTE | grep ');' |
> sed 's,.*\(".*%.*"\).*,\1,' | sort | uniq -c
>   1 "%08llx\n"
>   5 "0x%016llx\n"
>   5 "0x%02llx\n"
>   5 "0x%04llx\n"
>  13 "0x%08llx\n"
>   1 "0x%4.4llx\n"
>   3 "0x%.4llx\n"
>   4 "0x%llx\n"
>   1 "%1lld\n"
>  40 "%lld\n"
>   2 "%lli\n"
> 129 "%llu\n"
>   1 "%#llx\n"
>   2 "%llx\n"
> 
> u64 is not an issue for negative numbers.
> The %lld and %llu in any case are for 64-bit value, representing it as
> unsigned simplifies the generic code, but it doesn't mean we can't keep
> their signed value if we know that.
> 
> This change uses sscanf() to fix the problem since it does the conversion
> based on the supplied format string.
> 
> Fixes: 488dac0c9237 ("libfs: fix error cast of negative value in simple_attr_write()")

488dac0c9237 was two years ago, so I'm assuming that this error isn't
causing a lot of trouble out there.

In which I may be totally wrong.  Do you see a reason for backporting
this fix into earlier kernels?  If so, what is it?

Thanks.