linux-stable.git/arch/parisc/kernel/module.lds, branch v5.4 Linux kernel stable tree parisc: add dynamic ftrace 2019-06-08T10:56:29+00:00 Sven Schnelle svens@stackframe.org 2019-06-05T20:32:22+00:00 6ca6366220ed285e29ee22f4cf5c68a0397cb005 This patch implements dynamic ftrace for PA-RISC. The required mcount call sequences can get pretty long, so instead of patching the whole call sequence out of the functions, we are using -fpatchable-function-entry from gcc. This puts a configurable amount of NOPS before/at the start of the function. Taking do_sys_open() as example, which would look like this when the call is patched out: 1036b248: 08 00 02 40 nop 1036b24c: 08 00 02 40 nop 1036b250: 08 00 02 40 nop 1036b254: 08 00 02 40 nop 1036b258 <do_sys_open>: 1036b258: 08 00 02 40 nop 1036b25c: 08 03 02 41 copy r3,r1 1036b260: 6b c2 3f d9 stw rp,-14(sp) 1036b264: 08 1e 02 43 copy sp,r3 1036b268: 6f c1 01 00 stw,ma r1,80(sp) When ftrace gets enabled for this function the kernel will patch these NOPs to: 1036b248: 10 19 57 20 <address of ftrace> 1036b24c: 6f c1 00 80 stw,ma r1,40(sp) 1036b250: 48 21 3f d1 ldw -18(r1),r1 1036b254: e8 20 c0 02 bv,n r0(r1) 1036b258 <do_sys_open>: 1036b258: e8 3f 1f df b,l,n .-c,r1 1036b25c: 08 03 02 41 copy r3,r1 1036b260: 6b c2 3f d9 stw rp,-14(sp) 1036b264: 08 1e 02 43 copy sp,r3 1036b268: 6f c1 01 00 stw,ma r1,80(sp) So the first NOP in do_sys_open() will be patched to jump backwards into some minimal trampoline code which pushes a stackframe, saves r1 which holds the return address, loads the address of the real ftrace function, and branches to that location. For 64 Bit things are getting a bit more complicated (and longer) because we must make sure that the address of ftrace location is 8 byte aligned, and the offset passed to ldd for fetching the address is 8 byte aligned as well. Note that gcc has a bug which misplaces the function label, and needs a patch to make dynamic ftrace work. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90751 for details. Signed-off-by: Sven Schnelle <svens@stackframe.org> Signed-off-by: Helge Deller <deller@gmx.de> 
This patch implements dynamic ftrace for PA-RISC. The required mcount
call sequences can get pretty long, so instead of patching the
whole call sequence out of the functions, we are using
-fpatchable-function-entry from gcc. This puts a configurable amount of
NOPS before/at the start of the function. Taking do_sys_open() as example,
which would look like this when the call is patched out:

1036b248:       08 00 02 40     nop
1036b24c:       08 00 02 40     nop
1036b250:       08 00 02 40     nop
1036b254:       08 00 02 40     nop

1036b258 <do_sys_open>:
1036b258:       08 00 02 40     nop
1036b25c:       08 03 02 41     copy r3,r1
1036b260:       6b c2 3f d9     stw rp,-14(sp)
1036b264:       08 1e 02 43     copy sp,r3
1036b268:       6f c1 01 00     stw,ma r1,80(sp)

When ftrace gets enabled for this function the kernel will patch these
NOPs to:

1036b248:       10 19 57 20     <address of ftrace>
1036b24c:       6f c1 00 80     stw,ma r1,40(sp)
1036b250:       48 21 3f d1     ldw -18(r1),r1
1036b254:       e8 20 c0 02     bv,n r0(r1)

1036b258 <do_sys_open>:
1036b258:       e8 3f 1f df     b,l,n .-c,r1
1036b25c:       08 03 02 41     copy r3,r1
1036b260:       6b c2 3f d9     stw rp,-14(sp)
1036b264:       08 1e 02 43     copy sp,r3
1036b268:       6f c1 01 00     stw,ma r1,80(sp)

So the first NOP in do_sys_open() will be patched to jump backwards into
some minimal trampoline code which pushes a stackframe, saves r1 which
holds the return address, loads the address of the real ftrace function,
and branches to that location. For 64 Bit things are getting a bit more
complicated (and longer) because we must make sure that the address of
ftrace location is 8 byte aligned, and the offset passed to ldd for
fetching the address is 8 byte aligned as well.

Note that gcc has a bug which misplaces the function label, and needs a
patch to make dynamic ftrace work. See
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90751 for details.

Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>