Re: [Xen-devel] [PATCH v6 09/12] xen: add runtime parameter access support to hypfs

["Jürgen Groß" <jgross@suse.com>]

On 04.03.20 16:19, Jan Beulich wrote:
> On 04.03.2020 16:07, Jürgen Groß wrote:
>> On 04.03.20 12:32, Jan Beulich wrote:
>>> On 26.02.2020 13:47, Juergen Gross wrote:
>>>> +static void update_ept_param_append(const char *str, int val)
>>>> +{
>>>> +    char *pos = opt_ept_setting + strlen(opt_ept_setting);
>>>> +
>>>> +    snprintf(pos, sizeof(opt_ept_setting) - (pos - opt_ept_setting),
>>>> +             ",%s=%d", str, val);
>>>> +}
>>>> +
>>>> +static void update_ept_param(void)
>>>> +{
>>>> +    snprintf(opt_ept_setting, sizeof(opt_ept_setting), "pml=%d", opt_ept_pml);
>>>> +    if ( opt_ept_ad >= 0 )
>>>> +        update_ept_param_append("ad", opt_ept_ad);
>>>
>>> This won't correctly reflect reality: If you look at
>>> vmx_init_vmcs_config(), even a negative value means "true" here,
>>> unless on a specific Atom model. I think init_ept_param() wants
>>> to have that erratum workaround logic moved there, such that
>>> you can then assme the value to be non-negative here.
>>
>> But isn't not mentioning it in the -1 case correct? -1 means: do the
>> correct thing on the current hardware.
> 
> Well, I think the output here should represent effective settings,

The minimum requirement is to reflect the effective parameters, like
cmdline is doing for boot-time only parameters. With runtime parameters
we had no way of telling what was set, and this is now possible.

> and a sub-item should be suppressed only if a setting has no effect
> at all in the current setup, like ...
> 
>>>> +    if ( opt_ept_exec_sp >= 0 )
>>>> +        update_ept_param_append("exec-sp", opt_ept_exec_sp);
>>>
>>> I agree for this one - if the value is still -1, it has neither
>>> been set nor is its value of any interest.
> 
> ... here.

I think we should not mix up specified parameters and effective
settings. In case an effective setting is of common interest it should
be reported via a specific node (like e.g. specific mitigation settings
where the cmdline is not providing enough details).

> 
>>>> --- a/xen/common/grant_table.c
>>>> +++ b/xen/common/grant_table.c
>>>> @@ -85,8 +85,10 @@ struct grant_table {
>>>>        struct grant_table_arch arch;
>>>>    };
>>>>    
>>>> -static int parse_gnttab_limit(const char *param, const char *arg,
>>>> -                              unsigned int *valp)
>>>> +#define GRANT_CUSTOM_VAL_SZ  12
>>>> +
>>>> +static int parse_gnttab_limit(const char *arg, unsigned int *valp,
>>>> +                              char *parval)
>>>>    {
>>>>        const char *e;
>>>>        unsigned long val;
>>>> @@ -99,28 +101,47 @@ static int parse_gnttab_limit(const char *param, const char *arg,
>>>>            return -ERANGE;
>>>>    
>>>>        *valp = val;
>>>> +    snprintf(parval, GRANT_CUSTOM_VAL_SZ, "%lu", val);
>>>>    
>>>>        return 0;
>>>>    }
>>>>    
>>>>    unsigned int __read_mostly opt_max_grant_frames = 64;
>>>> +static char __read_mostly opt_max_grant_frames_val[GRANT_CUSTOM_VAL_SZ];
>>>> +
>>>> +static void __init gnttab_max_frames_init(struct param_hypfs *par)
>>>> +{
>>>> +    custom_runtime_set_var(par, opt_max_grant_frames_val);
>>>
>>> You still use a custom string buffer here. Can this "set-var"
>>> operation record that the variable (for presentation purposes)
>>> is simply of UINT type, handing a pointer to the actual
>>> variable?
>>
>> No, this would result in the need to set a custom parameter via a
>> binary value passed in from user land. So I'd need to convert this
>> binary into a string to be parseable by the custom function.
> 
> Hmm, not very fortunate, but I can see what you're saying.
> 
>>>> --- a/xen/common/hypfs.c
>>>> +++ b/xen/common/hypfs.c
>>>> @@ -10,6 +10,7 @@
>>>>    #include <xen/hypercall.h>
>>>>    #include <xen/hypfs.h>
>>>>    #include <xen/lib.h>
>>>> +#include <xen/param.h>
>>>>    #include <xen/rwlock.h>
>>>>    #include <public/hypfs.h>
>>>>    
>>>> @@ -281,6 +282,33 @@ int hypfs_write_bool(struct hypfs_entry_leaf *leaf,
>>>>        return 0;
>>>>    }
>>>>    
>>>> +int hypfs_write_custom(struct hypfs_entry_leaf *leaf,
>>>> +                       XEN_GUEST_HANDLE_PARAM(void) uaddr, unsigned long ulen)
>>>> +{
>>>> +    struct param_hypfs *p;
>>>> +    char *buf;
>>>> +    int ret;
>>>> +
>>>> +    buf = xzalloc_array(char, ulen);
>>>> +    if ( !buf )
>>>> +        return -ENOMEM;
>>>> +
>>>> +    ret = -EFAULT;
>>>> +    if ( copy_from_guest(buf, uaddr, ulen) )
>>>> +        goto out;
>>>> +
>>>> +    ret = -EDOM;
>>>> +    if ( !memchr(buf, 0, ulen) )
>>>
>>> Once again " != buf + ulen - 1"? (EDOM also looks like an odd
>>> error code to use in this case, but I guess there's no really
>>> good one.)
>>
>> " != buf + ulen - 1" is a logical choice with the change of patch 4.
> 
> I'm afraid I don't understand. You want to parse a string here.
> The caller should tell you what the string length is (including
> the nul again), not what its buffer size may be.

I agreed that changing to " != buf + ulen - 1" makes sense as I
agreed already to do so in patch 4.

> 
>>>> @@ -79,41 +88,94 @@ extern const struct kernel_param __param_start[], __param_end[];
>>>>              .type = OPT_IGNORE }
>>>>    
>>>>    #define __rtparam         __param(__dataparam)
>>>> +#define __paramfs         static __paramhypfs \
>>>> +    __attribute__((__aligned__(sizeof(void *)))) struct param_hypfs
>>>>    
>>>> -#define custom_runtime_only_param(_name, _var) \
>>>> +#define custom_runtime_set_var(parfs, var) \
>>>> +    { \
>>>> +        (parfs)->hypfs.write_ptr = &(var); \
>>>> +        (parfs)->hypfs.e.size = sizeof(var); \
>>>
>>> All users of this use char[]. Why sizeof() rather than strlen(),
>>
>> That is the maximum string length. Otherwise I wouldn't know I am
>> allowed to replace e.g. "on" by "noxpti".
> 
> As said elsewhere - if e.size is the buffer size, then the
> reading function wants adjusting, and it needs to be clarified
> how buffer size and payload size can be told apart for BLOBs.

Okay, I'll adjust the reading size to copy only strlen() + 1 bytes
and add a comment that BLOBs need blob-specific write and read
functions in the common case.


Juergen

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