Messages in this thread | ![/](/images/icornerl.gif) | | Date | Sun, 13 Jul 2008 11:36:02 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [PATCH] x86_64: fix delayed signals |
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On Sun, 13 Jul 2008, Oleg Nesterov wrote: > > SETFL_MASK doesn't have O_RDWR, and in any case setfl() changes ->f_flags, > not ->f_mode.
I think you can still read a write-only file descriptor on some systems: doing a mmap( PROT_WRITE ) on it can sometimes do it.
The reason - PROT_WRITE often implies PROT_READ (because on many CPU's you have to do unaligned writes with a read-modify-write cycle - on some like old alphas you have to do even normal byte and word writes that way).
So if the OS checks just the asked-for protections against the file descriptor protections, you'll essentially get read access for free when you do the mmap.
At least current versions of Linux won't ever allow a file mapping of a non-readable fd, but I won't guarantee we always did that, nor that other OS's always do it.
Anyway, I misunderstood Edwins idea to actually throw away the writes if the file isn't linked anywhere and is only open for writing. I guess we could optimize that, but it's such an unrealistic specialc case that it really isn't worth it except for some benchmark-gaming kind of thing (which I don't like doing anyway).
Linus
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