linux-kernel - Re: [PATCH v2] um: Allow building and running on older hosts

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Message-ID: <82371937-3efc-06e7-8394-2acbb8d38d3a@gmail.com>
Date:   Thu, 25 May 2017 15:28:18 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Thomas Meyer <thomas@...3r.de>
Cc:     linux-kernel@...r.kernel.org, elicooper@....com,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-devel@...ts.sourceforge.net>,
        "open list:USER-MODE LINUX (UML)" 
        <user-mode-linux-user@...ts.sourceforge.net>
Subject: Re: [PATCH v2] um: Allow building and running on older hosts

On 05/25/2017 12:51 PM, Thomas Meyer wrote:
> 
>> Am 25.05.2017 um 20:36 schrieb Florian Fainelli <f.fainelli@...il.com>:
>>
>> Commit a78ff1112263 ("um: add extended processor state save/restore
>> support") and b6024b21fec8 ("um: extend fpstate to _xstate to support
>> YMM registers") forced the use of the x86 FP _xstate and
>> PTRACE_GETREGSET/SETREGSET. On older hosts, we would neither be able to
>> build UML nor run it anymore with these two commits applied because we
>> don't have definitions for struct _xstate nor these two ptrace requests.
> 
> Btw. what soy you mean with older host? I need to revert two commits to make an UML kernel work again on a Fedora 25 installation.

This is centos 6.9, so kernel 2.6.32 and glibc 2.12, this neither has
xstate nor PTRACE_GETREGSET/SETREGSET, hence this patch.

> 
>>
>> We can determine at build time which fp context structure to check
>> against, just like we can keep using the old i387 fp save/restore if
>> PTRACE_GETRESET/SETREGSET are not defined.
>>
>> Fixes: a78ff1112263 ("um: add extended processor state save/restore support")
>> Fixes: b6024b21fec8 ("um: extend fpstate to _xstate to support YMM registers")
>> Signed-off-by: Florian Fainelli <f.fainelli@...il.com>
>> ---
>> Changes in v2:
>>
>> - re-arrange the #ifdef/curly braces to not take the branch
>>
>> arch/x86/um/os-Linux/registers.c | 12 ++++++++----
>> arch/x86/um/user-offsets.c       |  4 ++++
>> 2 files changed, 12 insertions(+), 4 deletions(-)
>>
>> diff --git a/arch/x86/um/os-Linux/registers.c b/arch/x86/um/os-Linux/registers.c
>> index 00f54a91bb4b..28775f55bde2 100644
>> --- a/arch/x86/um/os-Linux/registers.c
>> +++ b/arch/x86/um/os-Linux/registers.c
>> @@ -26,6 +26,7 @@ int save_i387_registers(int pid, unsigned long *fp_regs)
>>
>> int save_fp_registers(int pid, unsigned long *fp_regs)
>> {
>> +#ifdef PTRACE_GETREGSET
>>    struct iovec iov;
>>
>>    if (have_xstate_support) {
>> @@ -34,9 +35,9 @@ int save_fp_registers(int pid, unsigned long *fp_regs)
>>        if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>>            return -errno;
>>        return 0;
>> -    } else {
>> +    } else
>> +#endif
>>        return save_i387_registers(pid, fp_regs);
>> -    }
>> }
>>
>> int restore_i387_registers(int pid, unsigned long *fp_regs)
>> @@ -48,6 +49,7 @@ int restore_i387_registers(int pid, unsigned long *fp_regs)
>>
>> int restore_fp_registers(int pid, unsigned long *fp_regs)
>> {
>> +#ifdef PTRACE_SETREGSET
>>    struct iovec iov;
>>
>>    if (have_xstate_support) {
>> @@ -56,9 +58,9 @@ int restore_fp_registers(int pid, unsigned long *fp_regs)
>>        if (ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov) < 0)
>>            return -errno;
>>        return 0;
>> -    } else {
>> +    } else
>> +#endif
>>        return restore_i387_registers(pid, fp_regs);
>> -    }
>> }
>>
>> #ifdef __i386__
>> @@ -122,6 +124,7 @@ int put_fp_registers(int pid, unsigned long *regs)
>>
>> void arch_init_registers(int pid)
>> {
>> +#ifdef PTRACE_GETREGSET
>>    struct _xstate fp_regs;
>>    struct iovec iov;
>>
>> @@ -129,6 +132,7 @@ void arch_init_registers(int pid)
>>    iov.iov_len = sizeof(struct _xstate);
>>    if (ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov) == 0)
>>        have_xstate_support = 1;
> 
> In my setup above call works correctly and so have_xstate_support will be set, but save_fp_registers fails with errno 4 or so...
> 
>> +#endif
>> }
>> #endif
>>
>> diff --git a/arch/x86/um/user-offsets.c b/arch/x86/um/user-offsets.c
>> index cb3c22370cf5..8af0fb5d2780 100644
>> --- a/arch/x86/um/user-offsets.c
>> +++ b/arch/x86/um/user-offsets.c
>> @@ -50,7 +50,11 @@ void foo(void)
>>    DEFINE(HOST_GS, GS);
>>    DEFINE(HOST_ORIG_AX, ORIG_EAX);
>> #else
>> +#if defined(PTRACE_GETREGSET) && defined(PTRACE_SETREGSET)
>>    DEFINE(HOST_FP_SIZE, sizeof(struct _xstate) / sizeof(unsigned long));
>> +#else
>> +    DEFINE(HOST_FP_SIZE, sizeof(struct _fpstate) / sizeof(unsigned long));
>> +#endif
>>    DEFINE_LONGS(HOST_BX, RBX);
>>    DEFINE_LONGS(HOST_CX, RCX);
>>    DEFINE_LONGS(HOST_DI, RDI);
>> -- 
>> 2.12.2
>>


-- 
Florian