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Description
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The s_flag field in the snode structure (core data structure of specfs) is a ushort_t. With the integration of a recent bugfix, the last flag available (0x8000) has been taken, leaving no more space for other flags. Since drivers could potentially dereference an snode pointer (it is not opaque like a devinfo node), it may not be safe to expand the s_flag field as it will change the offset of subsequent fields. Instead we propose adding a new field at the end of the snode struct. This new field will be s_auxflag and will be a uint32_t.
Chris Horne pointed out that the s_flag field although a "short" is bracketed by two 4 byte(x86)/8 byte(sparcv9/amd64) fields (s_size and s_fsid) so there is likely to be some padding to meet alignment restrictions and/or the C "array" rule. Indeed a look via mdb shows the following
sparcv9
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> ::offsetof "struct snode" s_flag
offsetof (struct snode, s_flag) = 0x50
> ::offsetof "struct snode" s_fsid
offsetof (struct snode, s_fsid) = 0x58
i386
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> ::offsetof "struct snode" s_flag
offsetof (struct snode, s_flag) = 0x30
> ::offsetof "struct snode" s_fsid
offsetof (struct snode, s_fsid) = 0x34
amd64
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> ::offsetof "struct snode" s_flag
offsetof (struct snode, s_flag) = 0x50
> ::offsetof "struct snode" s_fsid
offsetof (struct snode, s_fsid) = 0x58
So to summarize, due to existing unused padding space, it is safe to extend the size of the s_flag field from a ushort_t to a uint_t. The offsets of subsequent fields will be unaffected by the extension of the s_flag field.
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