linux-hardening - [PATCH v2] [next] drm/amdgpu: Replace one-element array with flexible-array member

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Date:   Sat, 29 Oct 2022 14:30:44 +1300
From:   Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
To:     Alex Deucher <alexander.deucher@....com>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Hans de Goede <hdegoede@...hat.com>,
        Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>,
        Claudio Suarez <cssk@...-c.es>,
        Harry Wentland <harry.wentland@....com>,
        Grigory Vasilyev <h0tc0d3@...il.com>,
        Slark Xiao <slark_xiao@....com>,
        Rongguang Wei <weirongguang@...inos.cn>,
        amd-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Cc:     linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
        paulo.miguel.almeida.rodenas@...il.com
Subject: [PATCH v2] [next] drm/amdgpu: Replace one-element array with
 flexible-array member

One-element arrays are deprecated, and we are replacing them with
flexible array members instead. So, replace one-element array with
flexible-array member in struct _ATOM_FAKE_EDID_PATCH_RECORD and
refactor the rest of the code accordingly.

Important to mention is that doing a build before/after this patch
results in no binary output differences.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/238
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101836 [1]

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@...il.com>
---
Changelog:

v2: no binary output differences patch; report binary changes findings
    on commit log. Res: Kees Cook
v1: https://lore.kernel.org/lkml/Y1tkWdwPUp+UdpM0@mail.google.com/
---
 drivers/gpu/drm/amd/amdgpu/atombios_encoders.c | 7 +++++--
 drivers/gpu/drm/amd/include/atombios.h         | 2 +-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/atombios_encoders.c b/drivers/gpu/drm/amd/amdgpu/atombios_encoders.c
index 6be9ac2b9c5b..18ae9433e463 100644
--- a/drivers/gpu/drm/amd/amdgpu/atombios_encoders.c
+++ b/drivers/gpu/drm/amd/amdgpu/atombios_encoders.c
@@ -2081,8 +2081,11 @@ amdgpu_atombios_encoder_get_lcd_info(struct amdgpu_encoder *encoder)
 						}
 					}
 					record += fake_edid_record->ucFakeEDIDLength ?
-						fake_edid_record->ucFakeEDIDLength + 2 :
-						sizeof(ATOM_FAKE_EDID_PATCH_RECORD);
+						  struct_size(fake_edid_record,
+							      ucFakeEDIDString,
+							      fake_edid_record->ucFakeEDIDLength) :
+						  /* empty fake edid record must be 3 bytes long */
+						  sizeof(ATOM_FAKE_EDID_PATCH_RECORD) + 1;
 					break;
 				case LCD_PANEL_RESOLUTION_RECORD_TYPE:
 					panel_res_record = (ATOM_PANEL_RESOLUTION_PATCH_RECORD *)record;
diff --git a/drivers/gpu/drm/amd/include/atombios.h b/drivers/gpu/drm/amd/include/atombios.h
index 15943bc21bc5..b5b1d073f8e2 100644
--- a/drivers/gpu/drm/amd/include/atombios.h
+++ b/drivers/gpu/drm/amd/include/atombios.h
@@ -4107,7 +4107,7 @@ typedef struct _ATOM_FAKE_EDID_PATCH_RECORD
 {
   UCHAR ucRecordType;
   UCHAR ucFakeEDIDLength;       // = 128 means EDID length is 128 bytes, otherwise the EDID length = ucFakeEDIDLength*128
-  UCHAR ucFakeEDIDString[1];    // This actually has ucFakeEdidLength elements.
+  UCHAR ucFakeEDIDString[];     // This actually has ucFakeEdidLength elements.
 } ATOM_FAKE_EDID_PATCH_RECORD;
 
 typedef struct  _ATOM_PANEL_RESOLUTION_PATCH_RECORD
-- 
2.37.3