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f1fe2d21f4
The context introduced by MSA needs to be saved around signals. However, we can't increase the size of struct sigcontext because that will change the offset of the signal mask in struct sigframe or struct ucontext. This patch instead places the new context immediately after the struct sigframe for traditional signals, or similarly after struct ucontext for RT signals. The layout of struct sigframe & struct ucontext is identical from their sigcontext fields onwards, so the offset from the sigcontext to the extended context will always be the same regardless of the type of signal. Userland will be able to search through the extended context by using the magic values to detect which types of context are present. Any unrecognised context can be skipped over using the size field of struct extcontext. Once the magic value END_EXTCONTEXT_MAGIC is seen it is known that there are no further extended context structures to examine. This approach is somewhat similar to that taken by ARM to save VFP & other context at the end of struct ucontext. Userland can determine whether extended context is present by checking for the USED_EXTCONTEXT bit in the sc_used_math field of struct sigcontext. Whilst this could potentially change the historic semantics of sc_used_math if further extended context which does not imply FP context were to be introduced in the future, I have been unable to find any userland code making use of sc_used_math at all. Using one of the fields described as unused in struct sigcontext was considered, but the kernel does not already write to those fields so there would be no guarantee of the field being clear on older kernels. Other alternatives would be to have userland check the kernel version, or to have a HWCAP bit indicating presence of extended context. However there is a desire to have the context & information required to decode it be self contained such that, for example, debuggers could decode the saved context easily. [ralf@linux-mips.org: Fixed conflict.] Signed-off-by: Paul Burton <paul.burton@imgtec.com> Cc: linux-mips@linux-mips.org Cc: Guenter Roeck <linux@roeck-us.net> Cc: Matthew Fortune <matthew.fortune@imgtec.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com> Cc: Alex Smith <alex@alex-smith.me.uk> Cc: linux-kernel@vger.kernel.org Cc: Richard Weinberger <richard@nod.at> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Michael Ellerman <mpe@ellerman.id.au> Cc: Markos Chandras <markos.chandras@imgtec.com> Cc: Daniel Borkmann <dborkman@redhat.com> Cc: Maciej W. Rozycki <macro@codesourcery.com> Patchwork: https://patchwork.linux-mips.org/patch/10795/ Signed-off-by: Ralf Baechle <ralf@linux-mips.org> |
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