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Recently, some compile-time checking I added to the clamp_t family of
functions triggered a build error when a poorly written driver was
compiled on ARM, because the driver assumed that the naked `char` type
is signed, but ARM treats it as unsigned, and the C standard says it's
architecture-dependent.
I doubt this particular driver is the only instance in which
unsuspecting authors make assumptions about `char` with no `signed` or
`unsigned` specifier. We were lucky enough this time that that driver
used `clamp_t(char, negative_value, positive_value)`, so the new
checking code found it, and I've sent a patch to fix it, but there are
likely other places lurking that won't be so easily unearthed.
So let's just eliminate this particular variety of heisensign bugs
entirely. Set `-funsigned-char` globally, so that gcc makes the type
unsigned on all architectures.
This will break things in some places and fix things in others, so this
will likely cause a bit of churn while reconciling the type misuse.
Cc: Masahiro Yamada <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Andy Shevchenko <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Signed-off-by: Jason A. Donenfeld <[email protected]>
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