Laszlo,
I have been following this thread. I think the style
used here to access the 3 array elements to build the
24-bit size value is the best approach. I prefer this
over adding the union.
I agree there is a read overrun issue when using UINT32 to
read the Size[3] array contents.
I do not think this is a real issue in practice, because the
Size[3] array accessed is part of the larger
EFI_COMMON_SECTION_HEADER structure. However, we always should
clean up code to not do any read/write overruns without this
type of analysis and the need to keep track of exceptions.
There is a related set of code in the BaseLib for Read/Write
Unaligned24().
UINT32
EFIAPI
ReadUnaligned24 (
IN CONST UINT32 *Buffer
);
UINT32
EFIAPI
WriteUnaligned24 (
OUT UINT32 *Buffer,
IN UINT32 Value
);
This API does not get flagged for read overrun issues because
a UINT32 is passed in. However, for CPU archs that required aligned
access, the 24-bit value must be read in pieces. This is why there
are 2 different implementations:
IA32/X64
========
UINT32
EFIAPI
ReadUnaligned24 (
IN CONST UINT32 *Buffer
)
{
ASSERT (Buffer != NULL);
return *Buffer & 0xffffff;
}
ARM/AARCH64
============
UINT32
EFIAPI
ReadUnaligned24 (
IN CONST UINT32 *Buffer
)
{
ASSERT (Buffer != NULL);
return (UINT32)(
ReadUnaligned16 ((UINT16*)Buffer) |
(((UINT8*)Buffer)[2] << 16)
);
}
The ARM/ARCH64 implementation is clean because it does
not do a read overrun of the 24-bit field. The IA32/X64
implementation may have an issue because it reads a 32-bit
value and strips the upper 8 bits.
If we apply the same technique to the Size field of
EFI_COMMON_SECTION_HEADER, then the 24-bit value would be
built from reading only the 3 bytes of the array.
Best regards,
Mike
> -----Original Message-----
> From: [email protected] [mailto:[email protected]]
> On Behalf Of Laszlo Ersek
> Sent: Friday, April 12, 2019 4:31 PM
> To: edk2-devel-groups-io <[email protected]>
> Cc: Gao, Liming <[email protected]>; Kinney, Michael
> D <[email protected]>
> Subject: [edk2-devel] [PATCH 04/10]
> MdePkg/PiFirmwareFile: fix undefined behavior in
> FFS_FILE_SIZE
>
> Accessing "EFI_FFS_FILE_HEADER.Size", which is of type
> UINT8[3], through a
> (UINT32*), is undefined behavior. Fix it by accessing
> the array elements
> individually.
>
> (We can't use a union here, unfortunately, as easily as
> with
> "EFI_COMMON_SECTION_HEADER", given the fields in
> "EFI_FFS_FILE_HEADER".)
>
> Cc: Liming Gao <[email protected]>
> Cc: Michael D Kinney <[email protected]>
> Bugzilla:
> https://bugzilla.tianocore.org/show_bug.cgi?id=1710
> Signed-off-by: Laszlo Ersek <[email protected]>
> ---
> MdePkg/Include/Pi/PiFirmwareFile.h | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/MdePkg/Include/Pi/PiFirmwareFile.h
> b/MdePkg/Include/Pi/PiFirmwareFile.h
> index 4fce8298d1c0..0668f3fa9af4 100644
> --- a/MdePkg/Include/Pi/PiFirmwareFile.h
> +++ b/MdePkg/Include/Pi/PiFirmwareFile.h
> @@ -174,18 +174,26 @@ typedef struct {
> /// If FFS_ATTRIB_LARGE_FILE is not set then
> EFI_FFS_FILE_HEADER is used.
> ///
> UINT64 ExtendedSize;
> } EFI_FFS_FILE_HEADER2;
>
> #define IS_FFS_FILE2(FfsFileHeaderPtr) \
> (((((EFI_FFS_FILE_HEADER *) (UINTN)
> FfsFileHeaderPtr)->Attributes) & FFS_ATTRIB_LARGE_FILE)
> == FFS_ATTRIB_LARGE_FILE)
>
> +#define FFS_FILE_SIZE_ARRAY(FfsFileHeaderPtr) \
> + (((EFI_FFS_FILE_HEADER *) (UINTN)
> (FfsFileHeaderPtr))->Size)
> +
> +#define FFS_FILE_SIZE_ELEMENT(FfsFileHeaderPtr, Index)
> \
> + ((UINT32) FFS_FILE_SIZE_ARRAY
> (FfsFileHeaderPtr)[(Index)])
> +
> #define FFS_FILE_SIZE(FfsFileHeaderPtr) \
> - ((UINT32) (*((UINT32 *) ((EFI_FFS_FILE_HEADER *)
> (UINTN) FfsFileHeaderPtr)->Size) & 0x00ffffff))
> + ((FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 0) <<
> 0) | \
> + (FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 1) <<
> 8) | \
> + (FFS_FILE_SIZE_ELEMENT ((FfsFileHeaderPtr), 2) <<
> 16))
>
> #define FFS_FILE2_SIZE(FfsFileHeaderPtr) \
> ((UINT32) (((EFI_FFS_FILE_HEADER2 *) (UINTN)
> FfsFileHeaderPtr)->ExtendedSize))
>
> typedef UINT8 EFI_SECTION_TYPE;
>
> ///
> /// Pseudo type. It is used as a wild card when
> retrieving sections.
> --
> 2.19.1.3.g30247aa5d201
>
>
>
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