linux-kernel - Re: [PATCH 2/3] arm64/ptrace: introduce NT_ARM

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Date:   Wed, 27 Jan 2021 14:53:07 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Andrei Vagin <avagin@...il.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] arm64/ptrace: introduce NT_ARM_PRSTATUS to get a
 full set of registers

On Tue, Jan 19, 2021 at 02:06:36PM -0800, Andrei Vagin wrote:
> This is an alternative to NT_PRSTATUS that clobbers ip/r12 on AArch32,
> x7 on AArch64 when a tracee is stopped in syscall entry or syscall exit
> traps.
> 
> Signed-off-by: Andrei Vagin <avagin@...il.com>

This approach looks like it works, though I still think adding an option
for this under PTRACE_SETOPTIONS would be less intrusive.

Adding a shadow regset like this also looks like it would cause the gp
regs to be pointlessly be dumped twice in a core dump.  Avoiding that
might require hacks in the core code...


> ---
>  arch/arm64/kernel/ptrace.c | 39 ++++++++++++++++++++++++++++++++++++++
>  include/uapi/linux/elf.h   |  1 +
>  2 files changed, 40 insertions(+)
> 
> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> index 1863f080cb07..b8e4c2ddf636 100644
> --- a/arch/arm64/kernel/ptrace.c
> +++ b/arch/arm64/kernel/ptrace.c
> @@ -591,6 +591,15 @@ static int gpr_get(struct task_struct *target,
>  	return ret;
>  }
>  
> +static int gpr_get_full(struct task_struct *target,
> +		   const struct user_regset *regset,
> +		   struct membuf to)
> +{
> +	struct user_pt_regs *uregs = &task_pt_regs(target)->user_regs;
> +
> +	return membuf_write(&to, uregs, sizeof(*uregs));
> +}
> +
>  static int gpr_set(struct task_struct *target, const struct user_regset *regset,
>  		   unsigned int pos, unsigned int count,
>  		   const void *kbuf, const void __user *ubuf)
> @@ -1088,6 +1097,7 @@ static int tagged_addr_ctrl_set(struct task_struct *target, const struct
>  
>  enum aarch64_regset {
>  	REGSET_GPR,
> +	REGSET_GPR_FULL,

If we go with this approach, "REGSET_GPR_RAW" might be a preferable
name.  Both regs represent all the regs ("full"), but REGSET_GPR is
mangled by the kernel.

>  	REGSET_FPR,
>  	REGSET_TLS,
>  #ifdef CONFIG_HAVE_HW_BREAKPOINT
> @@ -1119,6 +1129,14 @@ static const struct user_regset aarch64_regsets[] = {
>  		.regset_get = gpr_get,
>  		.set = gpr_set
>  	},
> +	[REGSET_GPR_FULL] = {
> +		.core_note_type = NT_ARM_PRSTATUS,

Similarly, something like NT_ARM_PRSTATUS_RAW or similar.

> +		.n = sizeof(struct user_pt_regs) / sizeof(u64),
> +		.size = sizeof(u64),
> +		.align = sizeof(u64),
> +		.regset_get = gpr_get_full,
> +		.set = gpr_set
> +	},
>  	[REGSET_FPR] = {
>  		.core_note_type = NT_PRFPREG,
>  		.n = sizeof(struct user_fpsimd_state) / sizeof(u32),
> @@ -1225,6 +1243,7 @@ static const struct user_regset_view user_aarch64_view = {
>  #ifdef CONFIG_COMPAT
>  enum compat_regset {
>  	REGSET_COMPAT_GPR,
> +	REGSET_COMPAT_GPR_FULL,
>  	REGSET_COMPAT_VFP,
>  };
>  
> @@ -1285,6 +1304,18 @@ static int compat_gpr_get(struct task_struct *target,
>  	return 0;
>  }
>  
> +/* compat_gpr_get_full doesn't  overwrite x12 like compat_gpr_get. */
> +static int compat_gpr_get_full(struct task_struct *target,
> +			  const struct user_regset *regset,
> +			  struct membuf to)
> +{
> +	int i = 0;
> +
> +	while (to.left)
> +		membuf_store(&to, compat_get_user_reg(target, i++));
> +	return 0;
> +}
> +
>  static int compat_gpr_set(struct task_struct *target,
>  			  const struct user_regset *regset,
>  			  unsigned int pos, unsigned int count,
> @@ -1435,6 +1466,14 @@ static const struct user_regset aarch32_regsets[] = {
>  		.regset_get = compat_gpr_get,
>  		.set = compat_gpr_set
>  	},
> +	[REGSET_COMPAT_GPR_FULL] = {
> +		.core_note_type = NT_ARM_PRSTATUS,
> +		.n = COMPAT_ELF_NGREG,
> +		.size = sizeof(compat_elf_greg_t),
> +		.align = sizeof(compat_elf_greg_t),
> +		.regset_get = compat_gpr_get_full,
> +		.set = compat_gpr_set
> +	},
>  	[REGSET_COMPAT_VFP] = {
>  		.core_note_type = NT_ARM_VFP,
>  		.n = VFP_STATE_SIZE / sizeof(compat_ulong_t),
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index 30f68b42eeb5..a2086d19263a 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -426,6 +426,7 @@ typedef struct elf64_shdr {
>  #define NT_ARM_PACA_KEYS	0x407	/* ARM pointer authentication address keys */
>  #define NT_ARM_PACG_KEYS	0x408	/* ARM pointer authentication generic key */
>  #define NT_ARM_TAGGED_ADDR_CTRL	0x409	/* arm64 tagged address control (prctl()) */

What happened to 0x40a..0x40f?

[...]

Cheers
---Dave