From 94b07c1f8c39c6d839df35fa28ffd1785d385897 Mon Sep 17 00:00:00 2001 From: Dave Martin Date: Fri, 1 Jun 2018 11:10:14 +0100 Subject: arm64: signal: Report signal frame size to userspace via auxv MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Stateful CPU architecture extensions may require the signal frame to grow to a size that exceeds the arch's MINSIGSTKSZ #define. However, changing this #define is an ABI break. To allow userspace the option of determining the signal frame size in a more forwards-compatible way, this patch adds a new auxv entry tagged with AT_MINSIGSTKSZ, which provides the maximum signal frame size that the process can observe during its lifetime. If AT_MINSIGSTKSZ is absent from the aux vector, the caller can assume that the MINSIGSTKSZ #define is sufficient. This allows for a consistent interface with older kernels that do not provide AT_MINSIGSTKSZ. The idea is that libc could expose this via sysconf() or some similar mechanism. There is deliberately no AT_SIGSTKSZ. The kernel knows nothing about userspace's own stack overheads and should not pretend to know. For arm64: The primary motivation for this interface is the Scalable Vector Extension, which can require at least 4KB or so of extra space in the signal frame for the largest hardware implementations. To determine the correct value, a "Christmas tree" mode (via the add_all argument) is added to setup_sigframe_layout(), to simulate addition of all possible records to the signal frame at maximum possible size. If this procedure goes wrong somehow, resulting in a stupidly large frame layout and hence failure of sigframe_alloc() to allocate a record to the frame, then this is indicative of a kernel bug. In this case, we WARN() and no attempt is made to populate AT_MINSIGSTKSZ for userspace. For arm64 SVE: The SVE context block in the signal frame needs to be considered too when computing the maximum possible signal frame size. Because the size of this block depends on the vector length, this patch computes the size based not on the thread's current vector length but instead on the maximum possible vector length: this determines the maximum size of SVE context block that can be observed in any signal frame for the lifetime of the process. Signed-off-by: Dave Martin Acked-by: Will Deacon Cc: Ard Biesheuvel Cc: Alex Bennée Signed-off-by: Catalin Marinas --- arch/arm64/include/uapi/asm/auxvec.h | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) (limited to 'arch/arm64/include/uapi') diff --git a/arch/arm64/include/uapi/asm/auxvec.h b/arch/arm64/include/uapi/asm/auxvec.h index ec0a86d484e1..743c0b84fd30 100644 --- a/arch/arm64/include/uapi/asm/auxvec.h +++ b/arch/arm64/include/uapi/asm/auxvec.h @@ -19,7 +19,8 @@ /* vDSO location */ #define AT_SYSINFO_EHDR 33 +#define AT_MINSIGSTKSZ 51 /* stack needed for signal delivery */ -#define AT_VECTOR_SIZE_ARCH 1 /* entries in ARCH_DLINFO */ +#define AT_VECTOR_SIZE_ARCH 2 /* entries in ARCH_DLINFO */ #endif -- cgit v1.2.3