Messages in this thread | | | Subject | Re: [PATCH 02/17] prmem: write rare for static allocation | From | Igor Stoppa <> | Date | Mon, 29 Oct 2018 20:03:07 +0200 |
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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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