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Subjecta question regarding sys_poll() on x86_64 via tha ia32 layer
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Hello,

the ia32 poll system call is routed through the "standard" function
sys_poll().

This function is defined as:

SYSCALL_DEFINE3(poll, struct pollfd __user *, ufds, unsigned int, nfds,
long, timeout_msecs)

in fs/select.c

timeout_msecs is of type long which is AFAIK is 4 bytes on x86 and 8
bytes on x86_64.

the test for sign (i.e. < 0) in the objdump is done against the 64 bit
register (here %rbx):

ffffffff811313e0 <sys_poll>:
ffffffff811313e0: 55 push %rbp
ffffffff811313e1: 48 89 e5 mov %rsp,%rbp
ffffffff811313e4: 48 83 ec 30 sub $0x30,%rsp
ffffffff811313e8: 48 89 5d e8 mov %rbx,-0x18(%rbp)
ffffffff811313ec: 48 89 d3 mov %rdx,%rbx
ffffffff811313ef: 31 d2 xor %edx,%edx
ffffffff811313f1: 48 85 db test %rbx,%rbx
ffffffff811313f4: 4c 89 65 f0 mov %r12,-0x10(%rbp)
ffffffff811313f8: 4c 89 6d f8 mov %r13,-0x8(%rbp)
ffffffff811313fc: 41 89 f4 mov %esi,%r12d
ffffffff811313ff: 49 89 fd mov %rdi,%r13
ffffffff81131402: 78 42 js ffffffff81131446 <sys_poll+0x66>

on an x86 kernel the test is done against %ebx

so when the system call is called with %rbx = 00000000ffffffff (i.e. -1
from %ebx) on an x86_64 kernel via the ia32 layer the test for sign will
fail and the timer will be set.

btw. <sys/poll.h> seems to define the function as

extern int poll (struct pollfd *__fds, nfds_t __nfds, int __timeout);

what am I overloking?

mfg
thomas



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