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SubjectRe: [PATCH 02/17] prmem: write rare for static allocation
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On 25/10/2018 01:24, Dave Hansen wrote:
>> +static __always_inline bool __is_wr_after_init(const void *ptr, size_t size)
>> +{
>> + size_t start = (size_t)&__start_wr_after_init;
>> + size_t end = (size_t)&__end_wr_after_init;
>> + size_t low = (size_t)ptr;
>> + size_t high = (size_t)ptr + size;
>> +
>> + return likely(start <= low && low < high && high <= end);
>> +}
>
> size_t is an odd type choice for doing address arithmetic.

it seemed more portable than unsigned long

>> +/**
>> + * wr_memset() - sets n bytes of the destination to the c value
>> + * @dst: beginning of the memory to write to
>> + * @c: byte to replicate
>> + * @size: amount of bytes to copy
>> + *
>> + * Returns true on success, false otherwise.
>> + */
>> +static __always_inline
>> +bool wr_memset(const void *dst, const int c, size_t n_bytes)
>> +{
>> + size_t size;
>> + unsigned long flags;
>> + uintptr_t d = (uintptr_t)dst;
>> +
>> + if (WARN(!__is_wr_after_init(dst, n_bytes), WR_ERR_RANGE_MSG))
>> + return false;
>> + while (n_bytes) {
>> + struct page *page;
>> + uintptr_t base;
>> + uintptr_t offset;
>> + uintptr_t offset_complement;
>
> Again, these are really odd choices for types. vmap() returns a void*
> pointer, on which you can do arithmetic.

I wasn't sure of how much I could rely on the compiler not doing some
unwanted optimizations.

> Why bother keeping another
> type to which you have to cast to and from?

For the above reason. If I'm worrying unnecessarily, I can switch back
to void *
It certainly is easier to use.

> BTW, our usual "pointer stored in an integer type" is 'unsigned long',
> if a pointer needs to be manipulated.

yes, I noticed that, but it seemed strange ...
size_t corresponds to unsigned long, afaik

but it seems that I have not fully understood where to use it

anyway, I can stick to the convention with unsigned long

>
>> + local_irq_save(flags);
>
> Why are you doing the local_irq_save()?

The idea was to avoid the case where an attack would somehow freeze the
core doing the write-rare operation, while the temporary mapping is
accessible.

I have seen comments about using mappings that are private to the
current core (and I will reply to those comments as well), but this
approach seems architecture-dependent, while I was looking for a
solution that, albeit not 100% reliable, would work on any system with
an MMU. This would not prevent each arch to come up with own custom
implementation that provides better coverage, performance, etc.

>> + page = virt_to_page(d);
>> + offset = d & ~PAGE_MASK;
>> + offset_complement = PAGE_SIZE - offset;
>> + size = min(n_bytes, offset_complement);
>> + base = (uintptr_t)vmap(&page, 1, VM_MAP, PAGE_KERNEL);
>
> Can you even call vmap() (which sleeps) with interrupts off?

I accidentally disabled sleeping while atomic debugging and I totally
missed this problem :-(

However, to answer your question, nothing exploded while I was testing
(without that type of debugging).

I suspect I was just "lucky". Or maybe I was simply not triggering the
sleeping sub-case.

As I understood the code, sleeping _might_ happen, but it's not going to
happen systematically.

I wonder if I could split vmap() into two parts: first the sleeping one,
with interrupts enabled, then the non sleeping one, with interrupts
disabled.

I need to read the code more carefully, but it seems that sleeping might
happen when memory for the mapping meta data is not immediately available.

BTW, wouldn't the might_sleep() call belong more to the part which
really sleeps, instead than to the whole vmap() ?

>> + if (WARN(!base, WR_ERR_PAGE_MSG)) {
>> + local_irq_restore(flags);
>> + return false;
>> + }
>
> You really need some kmap_atomic()-style accessors to wrap this stuff
> for you. This little pattern is repeated over and over.

I really need to learn more about the way the kernel works and is
structured. It's a work in progress. Thanks for the advice.

> ...
>> +const char WR_ERR_RANGE_MSG[] = "Write rare on invalid memory range.";
>> +const char WR_ERR_PAGE_MSG[] = "Failed to remap write rare page.";
>
> Doesn't the compiler de-duplicate duplicated strings for you? Is there
> any reason to declare these like this?

I noticed I have made some accidental modifications in a couple of
cases, when replicating the command.

So I thought that if I really want to use the same string, why not doing
it explicitly? It seemed also easier, in case I want to tweak the
message. I need to do it only in one place.

--
igor

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