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SubjectRe: [PATCH] ptrace PTRACE_READDATA/WRITEDATA, kernel 2.5.62
fcorneli@elis.rug.ac.be wrote:

>But since the ptrace_readdata
>lives in the kernel tree for some time now and nobody is complaining about
>it I assume the sparc usage of ptrace_readdata is OK. I did test it on
>i386 and it works just fine.
>
>
Sorry, my fault. I remembered that someone must do double buffering.
I've overlooked that ptrace_readdata does the double buffering.

>When I look at the implementation of ptrace_readdata the dst (3th arg) has
>to be a pointer to user space; see: copy_to_user(dst, buf, retval). Only
>access_process_vm wants a kernel pointer. Anyway access_process_vm has
>some known issues, see:
>http://www.kernel.org/pub/linux/kernel/people/akpm/patches/2.5/2.5.62/2.5.62-mm3/broken-out/ptrace-flush.patch
>but it's getting fixed I hope.
>
>
This patch seems to be wrong.
flush_dcache_page is not a replacement for flush_page_to_ram(): It's an
optimized cache flush function, only valid for page cache pages:
If a page is mapped only once, then no aliasing can occur and the flush
is not required.
For page cache pages, "mapped once" is equivalient to an empty
page->mapping->i_mmap{,_shared}. The pages are mapped once in the page
cache, and _if_ they are mapped in user space, then
page->mapping->i_mmap{,_shared} is not empty.

ptrace can be called on random addresses - sysv shm, anonymous pages,
etc. page->mapping->i_mmap{,_shared} is meaningless.
If you think about it, if ptrace accesses a page, then it's guaranteed
to be mapped twice: once in user space, once by the kmap_atomic() for
the kernel space access.

--
Manfred



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