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    SubjectRe: [PATCH 1/2] tracing: Move syscalls metadata handling from arch to core
    On Sat, Sep 19, 2009 at 09:48:16AM +0200, Heiko Carstens wrote:
    > On Sat, Sep 19, 2009 at 07:39:16AM +0200, Frederic Weisbecker wrote:
    > > Most of the syscalls metadata processing is done from arch.
    > > But these operations are mostly generic accross archs. Especially now
    > > that we have a common variable name that expresses the number of
    > > syscalls supported by an arch: NR_syscalls, the only remaining bits
    > > that need to reside in arch is the syscall nr to addr translation.
    >
    > That won't work in its current form, since there is a small difference
    > between x86 and s390:
    >
    > > diff --git a/arch/s390/kernel/ftrace.c b/arch/s390/kernel/ftrace.c
    > > index 57bdcb1..7c5752c 100644
    > > --- a/arch/s390/kernel/ftrace.c
    > > +++ b/arch/s390/kernel/ftrace.c
    > > -static struct syscall_metadata *find_syscall_meta(unsigned long syscall)
    > > -{
    > > - struct syscall_metadata *start;
    > > - struct syscall_metadata *stop;
    > > - char str[KSYM_SYMBOL_LEN];
    > > -
    > > - start = (struct syscall_metadata *)__start_syscalls_metadata;
    > > - stop = (struct syscall_metadata *)__stop_syscalls_metadata;
    > > - kallsyms_lookup(syscall, NULL, NULL, NULL, str);
    > > -
    > > - for ( ; start < stop; start++) {
    > > - if (start->name && !strcmp(start->name + 3, str + 3))
    > ^^^^^^^^^^^^^^^^^^^^^^^^



    Oh, I thought it was to zap the "sys" prefix comparison.



    > > diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
    > > index 9dbb527..4adb867 100644
    > > --- a/arch/x86/kernel/ftrace.c
    > > +++ b/arch/x86/kernel/ftrace.c
    > > -static struct syscall_metadata *find_syscall_meta(unsigned long *syscall)
    > > -{
    > > - struct syscall_metadata *start;
    > > - struct syscall_metadata *stop;
    > > - char str[KSYM_SYMBOL_LEN];
    > > -
    > > -
    > > - start = (struct syscall_metadata *)__start_syscalls_metadata;
    > > - stop = (struct syscall_metadata *)__stop_syscalls_metadata;
    > > - kallsyms_lookup((unsigned long) syscall, NULL, NULL, NULL, str);
    > > -
    > > - for ( ; start < stop; start++) {
    > > - if (start->name && !strcmp(start->name, str))
    > ^^^^^^^^^^^^^^^^
    > > - return start;
    > > - }
    > > - return NULL;
    >
    > The reason for the "+ 3 " is that architectures with syscall wrappers have
    > alias function names which also show up in kallsysms:
    >
    > 000000000001c788 t show_cpuinfo
    > 000000000001c9e0 T SyS_s390_personality
    > 000000000001c9e0 T sys_s390_personality
    > 000000000001ca48 T SyS_s390_newuname
    > 000000000001ca48 T sys_s390_newuname
    > 000000000001cac8 T SyS_ipc
    > 000000000001cac8 T sys_ipc
    > 000000000001cd00 T SyS_s390_old_mmap
    > 000000000001cd00 T sys_s390_old_mmap
    > 000000000001ce68 T SyS_mmap2
    > 000000000001ce68 T sys_mmap2
    > 000000000001cfc4 t FixPerRegisters
    >
    > So kallsyms_lookup(...) would currently always return a string that starts
    > with "SyS" instead of "sys". Since the metadata syscall names start with
    > "sys" there is no match.
    > If you could change the generic version so it also contains a "+ 3" it
    > should work for all architectures.
    > Might be worth a comment that I didn't add back then :)



    Ah ok. So the fix is easy.
    Thanks for the tip!


    > Hmm... maybe it's even possible to throw out the "SyS" variants out of
    > the kallsyms table and only keep the alias names?
    > That would shrink the kernel image a bit.


    Yeah, unless someone finds strong reasons to keep them.

    Thanks!



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