Messages in this thread | | | Subject | 64-syscall args on 32-bit vs syscall() | From | Benjamin Herrenschmidt <> | Date | Mon, 15 Mar 2010 15:48:13 +1100 |
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Hoy there !
This may have been discussed earlier (I have some vague memories...) but I just hit a problem with that again (Mark: hint, it's in hdparm's fallocate) so I'd like a bit of a refresh here on what is the "right thing" to do...
So some syscalls want a 64-bit argument. Let's take fallocate() as our example. So we already know that we have to be extra careful since some 32-bit arch will pass this into 2 registers (or stack slots) which need to be aligned, and so we tend to already take care of making sure that the said 64-bit argument is either defined as 2x32-bit arguments, or defined as 1x64 bit argument aligned to 2x32-bit in the argument list.
So far so good...
The problem is when user space tries to use the same trick for calling those functions using glibc-provided syscall() function. In this example, hdparm does:
err = syscall(SYS_fallocate, fd, mode, offset, len);
With "offset" being a 64-bit argument.
This will break because the first argument to syscall now shifts everything by one register, which breaks the register pair alignment (and I suppose archs with stack based calling convention can have similar alignment issues even if x86 doesn't).
Ulrich, Steven, shouldn't we have glibc's syscall() take a long long as it's first argument to correct that ? Either that or making it some kind of macro wrapper around a __syscall(int dummy, int sysno, ...) ?
As it is, any 32-bit app using syscall() on any of the syscalls that takes 64-bit arguments will be broken, unless the app itself breaks up the argument, but the the order of the hi and lo part is different between BE and LE architectures ;-)
So is there a more "correct" solution than another here ? Should powerpc glibc be fixed at least so that syscall() keeps the alignment ?
Cheers, Ben.
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