Messages in this thread |  | | | From | Arnd Bergmann <> | | Subject | Re: [RFC][v5][PATCH 8/8]: Define clone_with_pids() syscall | | Date | Wed, 9 Sep 2009 14:19:50 +0200 |
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On Tuesday 08 September 2009, Nathan Lynch wrote:
> This doesn't work on a 64-bit kernel when the process is 32-bit and uses > the definition of struct pid_set provided in types.h: > > +struct pid_set { > + int num_pids; > + pid_t *pids; > +}; > > Shouldn't the pids field be u64 or some other type of fixed size?
This is a complex problem. The structure above would need a conversion for the pointer size that you can avoid by using a u64, but that introduces another problem:
struct pid_set { int num_pids; u64 pidp; }; Has implicit padding between the two members on all 64 bit architectures, but not on i386, so you would still need a conversion (not for s390, power, mips, sparc or parisc though, only for x86).
I can see two solutions for this:
1. use separate system call arguments for num_pids and pidp. This avoids the data structure and saves one copy_from_user call, at the cost of adding another argument to the syscall. syscalls with more than 6 arguments are somewhat problematic as well.
2. use a single pointer, with variable length data structures:
struct pid_set { int num_pids; pid_t pids[0]; }; Since pid_t is always an int, you have no problem with padding or incompatible types, but rely on a data structure definition that is not in C89 (not sure about C99).
Arnd <><
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