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Re: [Qemu-devel] [patch] firmware: qemu_fw_cfg.c: potential unintialized

From: Gabriel L. Somlo
Subject: Re: [Qemu-devel] [patch] firmware: qemu_fw_cfg.c: potential unintialized variable
Date: Thu, 14 Apr 2016 15:51:26 -0400
User-agent: Mutt/1.5.24 (2015-08-30)

On Thu, Apr 14, 2016 at 12:33:37PM +0300, Dan Carpenter wrote:
> It acpi_acquire_global_lock() return AE_NOT_CONFIGURED then "glk" isn't
> initialized, which, if you got very unlucky, could cause a bug.
> Signed-off-by: Dan Carpenter <address@hidden>
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> index d999fe3..0e20116 100644
> --- a/drivers/firmware/qemu_fw_cfg.c
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -77,7 +77,7 @@ static inline u16 fw_cfg_sel_endianness(u16 key)
>  static inline void fw_cfg_read_blob(u16 key,
>                                   void *buf, loff_t pos, size_t count)
>  {
> -     u32 glk;
> +     u32 glk = -1U;

After digging through the acpi_[acquire|release]_global_lock() code in
drivers/acpi/acpica/evxface.c, the -1 value actually makes sense, as
glk is set to the value of acpi_gbl_global_lock_handle, which
internally is a 16-bit value which can wrap around, but will never be
equal to 32-bit "-1". As such, the unlock function would fail with
AE_NOT_ACQUIRED if its "for-real" version ever ended up being called.

So, with the typos in the commit blurb fixed (s/It/If/ and
s/return/returns/), and on general "belt-and-suspenders" principle,

Reviewed-by: Gabriel Somlo <address@hidden>

I just wanted to make sure my understanding of "this can't happen with
the way the ACPI macros are currently defined" is still correct :)


>       acpi_status status;
>       /* If we have ACPI, ensure mutual exclusion against any potential

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