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    SubjectRe: [PATCH 11/11] [RFC] drm/i915/dp: fix array overflow warning
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    Hi,

    On 3/22/21 5:02 PM, Arnd Bergmann wrote:
    > From: Arnd Bergmann <arnd@arndb.de>
    >
    > gcc-11 warns that intel_dp_check_mst_status() has a local array of
    > fourteen bytes and passes the last four bytes into a function that
    > expects a six-byte array:
    >
    > drivers/gpu/drm/i915/display/intel_dp.c: In function ‘intel_dp_check_mst_status’:
    > drivers/gpu/drm/i915/display/intel_dp.c:4556:22: error: ‘drm_dp_channel_eq_ok’ reading 6 bytes from a region of size 4 [-Werror=stringop-overread]
    > 4556 | !drm_dp_channel_eq_ok(&esi[10], intel_dp->lane_count)) {
    > | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    > drivers/gpu/drm/i915/display/intel_dp.c:4556:22: note: referencing argument 1 of type ‘const u8 *’ {aka ‘const unsigned char *’}
    > In file included from drivers/gpu/drm/i915/display/intel_dp.c:38:
    > include/drm/drm_dp_helper.h:1459:6: note: in a call to function ‘drm_dp_channel_eq_ok’
    > 1459 | bool drm_dp_channel_eq_ok(const u8 link_status[DP_LINK_STATUS_SIZE],
    > | ^~~~~~~~~~~~~~~~~~~~
    >
    > Clearly something is wrong here, but I can't quite figure out what.
    > Changing the array size to 16 bytes avoids the warning, but is
    > probably the wrong solution here.

    The drm displayport-helpers indeed expect a 6 bytes buffer, but they
    usually only consume 4 bytes.

    I don't think that changing the DP_DPRX_ESI_LEN is a good fix here,
    since it is used in multiple places, but the esi array already gets
    zero-ed out by its initializer, so we can just pass 2 extra 0 bytes
    to give drm_dp_channel_eq_ok() call the 6 byte buffer its prototype
    specifies by doing this:

    diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
    index 897711d9d7d3..147962d4ad06 100644
    --- a/drivers/gpu/drm/i915/display/intel_dp.c
    +++ b/drivers/gpu/drm/i915/display/intel_dp.c
    @@ -4538,7 +4538,11 @@ intel_dp_check_mst_status(struct intel_dp *intel_dp)
    drm_WARN_ON_ONCE(&i915->drm, intel_dp->active_mst_links < 0);

    for (;;) {
    - u8 esi[DP_DPRX_ESI_LEN] = {};
    + /*
    + * drm_dp_channel_eq_ok() expects a 6 byte large buffer, but
    + * the ESI info only contains 4 bytes, pass 2 extra 0 bytes.
    + */
    + u8 esi[DP_DPRX_ESI_LEN + 2] = {};
    bool handled;
    int retry;

    So i915 devs, would such a fix be acceptable ?

    Regards,

    Hans






    >
    > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
    > ---
    > drivers/gpu/drm/i915/display/intel_dp.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
    > index 8c12d5375607..830e2515f119 100644
    > --- a/drivers/gpu/drm/i915/display/intel_dp.c
    > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
    > @@ -65,7 +65,7 @@
    > #include "intel_vdsc.h"
    > #include "intel_vrr.h"
    >
    > -#define DP_DPRX_ESI_LEN 14
    > +#define DP_DPRX_ESI_LEN 16
    >
    > /* DP DSC throughput values used for slice count calculations KPixels/s */
    > #define DP_DSC_PEAK_PIXEL_RATE 2720000
    >

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