Re: [PATCH v3 04/10] riscv: Add support for userspace pointer masking

[Samuel Holland]

Hi Anup,

On 2024-08-16 8:21 AM, Anup Patel wrote:
> On Wed, Aug 14, 2024 at 1:45 PM Samuel Holland
> <samuel.holland@xxxxxxxxxx> wrote:
>>
>> RISC-V supports pointer masking with a variable number of tag bits
>> (which is called "PMLEN" in the specification) and which is configured
>> at the next higher privilege level.
>>
>> Wire up the PR_SET_TAGGED_ADDR_CTRL and PR_GET_TAGGED_ADDR_CTRL prctls
>> so userspace can request a lower bound on the number of tag bits and
>> determine the actual number of tag bits. As with arm64's
>> PR_TAGGED_ADDR_ENABLE, the pointer masking configuration is
>> thread-scoped, inherited on clone() and fork() and cleared on execve().
>>
>> Signed-off-by: Samuel Holland <samuel.holland@xxxxxxxxxx>
>> ---
>>
>> Changes in v3:
>>  - Rename CONFIG_RISCV_ISA_POINTER_MASKING to CONFIG_RISCV_ISA_SUPM,
>>    since it only controls the userspace part of pointer masking
>>  - Use IS_ENABLED instead of #ifdef when possible
>>  - Use an enum for the supported PMLEN values
>>  - Simplify the logic in set_tagged_addr_ctrl()
>>
>> Changes in v2:
>>  - Rebase on riscv/linux.git for-next
>>  - Add and use the envcfg_update_bits() helper function
>>  - Inline flush_tagged_addr_state()
>>
>>  arch/riscv/Kconfig                 | 11 ++++
>>  arch/riscv/include/asm/processor.h |  8 +++
>>  arch/riscv/include/asm/switch_to.h | 11 ++++
>>  arch/riscv/kernel/process.c        | 90 ++++++++++++++++++++++++++++++
>>  include/uapi/linux/prctl.h         |  3 +
>>  5 files changed, 123 insertions(+)
>>
>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> index 0f3cd7c3a436..817437157138 100644
>> --- a/arch/riscv/Kconfig
>> +++ b/arch/riscv/Kconfig
>> @@ -512,6 +512,17 @@ config RISCV_ISA_C
>>
>>           If you don't know what to do here, say Y.
>>
>> +config RISCV_ISA_SUPM
>> +       bool "Supm extension for userspace pointer masking"
>> +       depends on 64BIT
>> +       default y
>> +       help
>> +         Add support for pointer masking in userspace (Supm) when the
>> +         underlying hardware extension (Smnpm or Ssnpm) is detected at boot.
>> +
>> +         If this option is disabled, userspace will be unable to use
>> +         the prctl(PR_{SET,GET}_TAGGED_ADDR_CTRL) API.
>> +
>>  config RISCV_ISA_SVNAPOT
>>         bool "Svnapot extension support for supervisor mode NAPOT pages"
>>         depends on 64BIT && MMU
>> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
>> index 586e4ab701c4..5c4d4fb97314 100644
>> --- a/arch/riscv/include/asm/processor.h
>> +++ b/arch/riscv/include/asm/processor.h
>> @@ -200,6 +200,14 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
>>  #define RISCV_SET_ICACHE_FLUSH_CTX(arg1, arg2) riscv_set_icache_flush_ctx(arg1, arg2)
>>  extern int riscv_set_icache_flush_ctx(unsigned long ctx, unsigned long per_thread);
>>
>> +#ifdef CONFIG_RISCV_ISA_SUPM
>> +/* PR_{SET,GET}_TAGGED_ADDR_CTRL prctl */
>> +long set_tagged_addr_ctrl(struct task_struct *task, unsigned long arg);
>> +long get_tagged_addr_ctrl(struct task_struct *task);
>> +#define SET_TAGGED_ADDR_CTRL(arg)      set_tagged_addr_ctrl(current, arg)
>> +#define GET_TAGGED_ADDR_CTRL()         get_tagged_addr_ctrl(current)
>> +#endif
>> +
>>  #endif /* __ASSEMBLY__ */
>>
>>  #endif /* _ASM_RISCV_PROCESSOR_H */
>> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
>> index 9685cd85e57c..94e33216b2d9 100644
>> --- a/arch/riscv/include/asm/switch_to.h
>> +++ b/arch/riscv/include/asm/switch_to.h
>> @@ -70,6 +70,17 @@ static __always_inline bool has_fpu(void) { return false; }
>>  #define __switch_to_fpu(__prev, __next) do { } while (0)
>>  #endif
>>
>> +static inline void envcfg_update_bits(struct task_struct *task,
>> +                                     unsigned long mask, unsigned long val)
>> +{
>> +       unsigned long envcfg;
>> +
>> +       envcfg = (task->thread.envcfg & ~mask) | val;
>> +       task->thread.envcfg = envcfg;
>> +       if (task == current)
>> +               csr_write(CSR_ENVCFG, envcfg);
>> +}
>> +
>>  static inline void __switch_to_envcfg(struct task_struct *next)
>>  {
>>         asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0",
>> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
>> index e4bc61c4e58a..1280a7c4a412 100644
>> --- a/arch/riscv/kernel/process.c
>> +++ b/arch/riscv/kernel/process.c
>> @@ -7,6 +7,7 @@
>>   * Copyright (C) 2017 SiFive
>>   */
>>
>> +#include <linux/bitfield.h>
>>  #include <linux/cpu.h>
>>  #include <linux/kernel.h>
>>  #include <linux/sched.h>
>> @@ -171,6 +172,9 @@ void flush_thread(void)
>>         memset(&current->thread.vstate, 0, sizeof(struct __riscv_v_ext_state));
>>         clear_tsk_thread_flag(current, TIF_RISCV_V_DEFER_RESTORE);
>>  #endif
>> +       if (IS_ENABLED(CONFIG_RISCV_ISA_SUPM) &&
>> +           riscv_has_extension_unlikely(RISCV_ISA_EXT_SUPM))
>> +               envcfg_update_bits(current, ENVCFG_PMM, ENVCFG_PMM_PMLEN_0);
> 
> Seeing a compile warning with this patch on RV32.
> 
> linux/arch/riscv/kernel/process.c: In function 'flush_thread':
> linux/arch/riscv/include/asm/csr.h:202:41: warning: conversion from
> 'long long unsigned int' to 'long unsigned int' changes value from
> '12884901888' to '0' [-Woverflow]
>   202 | #define ENVCFG_PMM                      (_AC(0x3, ULL) << 32)
>       |                                         ^~~~~~~~~~~~~~~~~~~~~
> linux/arch/riscv/kernel/process.c:179:45: note: in expansion of macro
> 'ENVCFG_PMM'
>   179 |                 envcfg_update_bits(current, ENVCFG_PMM,
> ENVCFG_PMM_PMLEN_0);
>       |                                             ^~~~~~~~~~

Right, thanks, that's why I needed to use #ifdef here before. I'll switch this
instance back for v4.

Regards,
Samuel