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Subject[patch] arch-specific cond_syscall usage issues
Experimenting with trying to use cond_syscall for a few arch-specific
syscalls, I discovered that it can't actually be used outside the file
in which sys_ni_syscall is declared because the assembler doesn't feel
obliged to output the symbol in that case:


#define cond_syscall(x) asm(".weak\t" #x "\n\t.set\t" #x ",sys_ni_syscall");

$ nm weak.o
U sys_ni_syscall

One arch (PPC) is apparently trying to use cond_syscall this way
anyway, though it's probably never been actually tested as the above
test was done on a PPC.

After trying a bunch of tricks to get it to work nicely, I decided
there are basically two alternatives: make weak versions of
sys_ni_syscall wherever they're wanted or put the arch-specific
cond_syscalls in kernel/sys.c where sys_ni_syscall is defined.

The former approach is a bit crufty and doesn't actually do the right
thing in practice as you'll get multiple copies of sys_ni_syscall in
your final image.

The latter introduces some slight arch-pollution in sys.c, but as
arch-specific cond_syscalls aren't all that frequent, it should be
pretty minor. So here's a patch to move the current offender to sys.c:

tiny-mpm/arch/ppc/kernel/syscalls.c | 2 --
tiny-mpm/kernel/sys.c | 3 +++
2 files changed, 3 insertions(+), 2 deletions(-)

diff -puN arch/ppc/kernel/syscalls.c~ppc_cond_syscall arch/ppc/kernel/syscalls.c
--- tiny/arch/ppc/kernel/syscalls.c~ppc_cond_syscall 2004-01-09 21:15:02.000000000 -0600
+++ tiny-mpm/arch/ppc/kernel/syscalls.c 2004-01-09 21:15:08.000000000 -0600
@@ -271,5 +271,3 @@ long ppc_fadvise64_64(int fd, int advice
return sys_fadvise64_64(fd, offset, len, advice);
diff -puN kernel/sys.c~ppc_cond_syscall kernel/sys.c
--- tiny/kernel/sys.c~ppc_cond_syscall 2004-01-09 21:15:02.000000000 -0600
+++ tiny-mpm/kernel/sys.c 2004-01-09 21:15:02.000000000 -0600
@@ -252,6 +252,9 @@ cond_syscall(sys_epoll_wait)

+/* arch-specific weak syscall entries */
static int set_one_prio(struct task_struct *p, int niceval, int error)
int no_nice;

Matt Mackall : : Linux development and consulting
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