Hi David,
Sorry for a late response, please find answers in-lined:
> > If this parameter is omitted, kernel behavior remains the same as it
> > was before this patch is applied.
>
> Do other architectures have a similar hack?
>
> If arm{,64} and x86 don't need this, what would make MIPS so special
> that we have to carry this around?
Yes, there are similar workarounds. Just a couple lines above
nonxstack description in the documentation there are :
noexec [IA-64]
noexec [X86]
On X86-32 available only on PAE configured kernels.
noexec=on: enable non-executable mappings (default)
noexec=off: disable non-executable mappings
..
noexec32 [X86-64]
This affects only 32-bit executables.
noexec32=on: enable non-executable mappings (default)
read doesn't imply executable mappings
noexec32=off: disable non-executable mappings
read implies executable mappings
> >
> > This functionality is convenient during debugging and is especially
> > useful for Android development where non-exec stack is required.
>
> Why not just set the PT_GNU_STACK flags correctly in the first place?
We do have PT_GNU_STACK flags set correctly, this feature is required to
workaround CPU revisions which do not have RIXI support.
Kind regards,
Miodrag
________________________________________
From: David Daney [ddaney@xxxxxxxxxxxxxxxxxx]
Sent: Tuesday, November 21, 2017 9:53 PM
To: Aleksandar Markovic; linux-mips@xxxxxxxxxxxxxx
Cc: Miodrag Dinic; Aleksandar Markovic; Andrew Morton; DengCheng Zhu; Ding Tianhong; Douglas Leung; Frederic Weisbecker; Goran Ferenc; Ingo Molnar; James Cowgill; James Hogan; Jonathan Corbet; linux-doc@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Marc Zyngier; Matt Redfearn; Mimi Zohar; Paul Burton; Paul E. McKenney; Petar Jovanovic; Raghu Gandham; Ralf Baechle; Thomas Gleixner; Tom Saeger
Subject: Re: [PATCH v2] MIPS: Add nonxstack=on|off kernel parameter
On 11/21/2017 05:56 AM, Aleksandar Markovic wrote:
> From: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
>
> Add a new kernel parameter to override the default behavior related
> to the decision whether to set up stack as non-executable in function
> mips_elf_read_implies_exec().
>
> The new parameter is used to control non executable stack and heap,
> regardless of PT_GNU_STACK entry. This does apply to both stack and
> heap, despite the name.
>
> Allowed values:
>
> nonxstack=on Force non-exec stack & heap
> nonxstack=off Force executable stack & heap
>
> If this parameter is omitted, kernel behavior remains the same as it
> was before this patch is applied.
Do other architectures have a similar hack?
If arm{,64} and x86 don't need this, what would make MIPS so special
that we have to carry this around?
>
> This functionality is convenient during debugging and is especially
> useful for Android development where non-exec stack is required.
Why not just set the PT_GNU_STACK flags correctly in the first place?
>
> Signed-off-by: Miodrag Dinic <miodrag.dinic@xxxxxxxx>
> Signed-off-by: Aleksandar Markovic <aleksandar.markovic@xxxxxxxx>
> ---
> Documentation/admin-guide/kernel-parameters.txt | 11 +++++++
> arch/mips/kernel/elf.c | 39 +++++++++++++++++++++++++
> 2 files changed, 50 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b74e133..99464ee 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2614,6 +2614,17 @@
> noexec32=off: disable non-executable mappings
> read implies executable mappings
>
> + nonxstack [MIPS]
> + Force setting up stack and heap as non-executable or
> + executable regardless of PT_GNU_STACK entry. Both
> + stack and heap are affected, despite the name. Valid
> + arguments: on, off.
> + nonxstack=on: Force non-executable stack and heap
> + nonxstack=off: Force executable stack and heap
> + If ommited, stack and heap will or will not be set
> + up as non-executable depending on PT_GNU_STACK
> + entry and possibly other factors.
> +
> nofpu [MIPS,SH] Disable hardware FPU at boot time.
>
> nofxsr [BUGS=X86-32] Disables x86 floating point extended
> diff --git a/arch/mips/kernel/elf.c b/arch/mips/kernel/elf.c
> index 731325a..28ef7f3 100644
> --- a/arch/mips/kernel/elf.c
> +++ b/arch/mips/kernel/elf.c
> @@ -326,8 +326,47 @@ void mips_set_personality_nan(struct arch_elf_state *state)
> }
> }
>
> +static int nonxstack = EXSTACK_DEFAULT;
> +
> +/*
> + * kernel parameter: nonxstack=on|off
> + *
> + * Force setting up stack and heap as non-executable or
> + * executable regardless of PT_GNU_STACK entry. Both
> + * stack and heap are affected, despite the name. Valid
> + * arguments: on, off.
> + *
> + * nonxstack=on: Force non-executable stack and heap
> + * nonxstack=off: Force executable stack and heap
> + *
> + * If ommited, stack and heap will or will not be set
> + * up as non-executable depending on PT_GNU_STACK
> + * entry and possibly other factors.
> + */
> +static int __init nonxstack_setup(char *str)
> +{
> + if (!strcmp(str, "on"))
> + nonxstack = EXSTACK_DISABLE_X;
> + else if (!strcmp(str, "off"))
> + nonxstack = EXSTACK_ENABLE_X;
> + else
> + pr_err("Malformed nonxstack format! nonxstack=on|off\n");
> +
> + return 1;
> +}
> +__setup("nonxstack=", nonxstack_setup);
> +
> int mips_elf_read_implies_exec(void *elf_ex, int exstack)
> {
> + switch (nonxstack) {
> + case EXSTACK_DISABLE_X:
> + return 0;
> + case EXSTACK_ENABLE_X:
> + return 1;
> + default:
> + break;
> + }
> +
> if (exstack != EXSTACK_DISABLE_X) {
> /* The binary doesn't request a non-executable stack */
> return 1;
>
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