Re: [PATCH v3 7/8] aarch64: Add sysv specific enabling code for memory tagging

[Siddhesh Poyarekar <siddhesh@gotplt.org>]

On 11/23/20 9:12 PM, Richard Earnshaw via Libc-alpha wrote:
> 
> Add various defines and stubs for enabling MTE on AArch64 sysv-like
> systems such as Linux.  The HWCAP feature bit is copied over in the
> same way as other feature bits.  Similarly we add a new wrapper header
> for mman.h to define the PROT_MTE flag that can be used with mmap and
> related functions.
> 
> We add a new field to struct cpu_features that can be used, for
> example, to check whether or not certain ifunc'd routines should be
> bound to MTE-safe versions.
> 
> Finally, if we detect that MTE should be enabled (ie via the glibc
> tunable); we enable MTE during startup as required.
> ---
>   sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h  |  1 +
>   sysdeps/unix/sysv/linux/aarch64/bits/mman.h   |  7 +++++
>   .../unix/sysv/linux/aarch64/cpu-features.c    | 28 +++++++++++++++++++
>   .../unix/sysv/linux/aarch64/cpu-features.h    |  1 +
>   4 files changed, 37 insertions(+)
> 
> diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
> index af90d8a626..389852f1d9 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
> +++ b/sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h
> @@ -73,3 +73,4 @@
>  #define HWCAP2_DGH		(1 << 15)
>  #define HWCAP2_RNG		(1 << 16)
>  #define HWCAP2_BTI		(1 << 17)
> +#define HWCAP2_MTE		(1 << 18)
> diff --git a/sysdeps/unix/sysv/linux/aarch64/bits/mman.h b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
> index ecae046344..3658b958b5 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
> +++ b/sysdeps/unix/sysv/linux/aarch64/bits/mman.h
> @@ -1,4 +1,5 @@
>  /* Definitions for POSIX memory map interface.  Linux/AArch64 version.
> +

Unnecessary newline.  I spotted a couple in 3/8 as well.

>     Copyright (C) 2020 Free Software Foundation, Inc.
>     This file is part of the GNU C Library.
>  
> @@ -25,6 +26,12 @@
>  
>  #define PROT_BTI	0x10
>  
> +/* The following definitions basically come from the kernel headers.
> +   But the kernel header is not namespace clean.  */
> +
> +/* Other flags.  */
> +#define PROT_MTE	0x20		/* Normal Tagged mapping.  */
> +
>  #include <bits/mman-map-flags-generic.h>
>  
>  /* Include generic Linux declarations.  */
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> index b9ab827aca..aa4a82c6e8 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> @@ -19,6 +19,7 @@
>  #include <cpu-features.h>
>  #include <sys/auxv.h>
>  #include <elf/dl-hwcaps.h>
> +#include <sys/prctl.h>
>  
>  #define DCZID_DZP_MASK (1 << 4)
>  #define DCZID_BS_MASK (0xf)
> @@ -86,4 +87,31 @@ init_cpu_features (struct cpu_features *cpu_features)
>  
>    /* Check if BTI is supported.  */
>    cpu_features->bti = GLRO (dl_hwcap2) & HWCAP2_BTI;
> +
> +  /* Setup memory tagging support if the HW and kernel support it, and if
> +     the user has requested it.  */
> +  cpu_features->mte_state = 0;
> +
> +#ifdef _LIBC_MTAG
> +# if HAVE_TUNABLES
> +  int mte_state = TUNABLE_GET (glibc, memtag, enable, unsigned, 0);
> +  cpu_features->mte_state = (GLRO (dl_hwcap2) & HWCAP2_MTE) ? mte_state : 0;
> +  /* If we lack the MTE feature, disable the tunable, since it will
> +     otherwise cause instructions that won't run on this CPU to be used.  */
> +  TUNABLE_SET (glibc, memtag, enable, unsigned, cpu_features->mte_state);
> +# endif
> +
> +  /* For now, disallow tag 0, so that we can clearly see when tagged
> +     addresses are being allocated.  */
> +  if (cpu_features->mte_state & 2)
> +    __prctl (PR_SET_TAGGED_ADDR_CTRL,
> +	     (PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC
> +	      | (0xfffe << PR_MTE_TAG_SHIFT)),

Couldn't this magic number also become a macro too?

> +	     0, 0, 0);
> +  else if (cpu_features->mte_state)
> +    __prctl (PR_SET_TAGGED_ADDR_CTRL,
> +	     (PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_ASYNC
> +	      | (0xfffe << PR_MTE_TAG_SHIFT)),

Likewise.

> +	     0, 0, 0);
> +#endif
>  }
> diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
> index 00a4d0c8e7..838d5c9aba 100644
> --- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
> +++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.h
> @@ -70,6 +70,7 @@ struct cpu_features
>    uint64_t midr_el1;
>    unsigned zva_size;
>    bool bti;
> +  unsigned mte_state;

This could be just a byte unless you foresee expanding the MTE flags 
beyond the 8 bits you've specified in the tunables.

>  };
>  
>  #endif /* _CPU_FEATURES_AARCH64_H  */