Teach ftrace_make_call() and ftrace_make_nop() about PLTs.
Teach PLT code about FTRACE and all its callbacks.
Otherwise the following might happen:

------------[ cut here ]------------
WARNING: CPU: 14 PID: 2265 at .../arch/arm/kernel/insn.c:14 
__arm_gen_branch+0x83/0x8c()
...
Hardware name: LSI Axxia AXM55XX
[<c0314a49>] (unwind_backtrace) from [<c03115e9>] (show_stack+0x11/0x14)
[<c03115e9>] (show_stack) from [<c0519f51>] (dump_stack+0x81/0xa8)
[<c0519f51>] (dump_stack) from [<c032185d>] (warn_slowpath_common+0x69/0x90)
[<c032185d>] (warn_slowpath_common) from [<c03218f3>] 
(warn_slowpath_null+0x17/0x1c)
[<c03218f3>] (warn_slowpath_null) from [<c03143cf>] (__arm_gen_branch+0x83/0x8c)
[<c03143cf>] (__arm_gen_branch) from [<c0314337>] (ftrace_make_nop+0xf/0x24)
[<c0314337>] (ftrace_make_nop) from [<c038ebcb>] 
(ftrace_process_locs+0x27b/0x3e8)
[<c038ebcb>] (ftrace_process_locs) from [<c0378d79>] (load_module+0x11e9/0x1a44)
[<c0378d79>] (load_module) from [<c037974d>] (SyS_finit_module+0x59/0x84)
[<c037974d>] (SyS_finit_module) from [<c030e981>] (ret_fast_syscall+0x1/0x18)
---[ end trace e1b64ced7a89adcc ]---
------------[ cut here ]------------
WARNING: CPU: 14 PID: 2265 at .../kernel/trace/ftrace.c:1979 
ftrace_bug+0x1b1/0x234()
...
Hardware name: LSI Axxia AXM55XX
[<c0314a49>] (unwind_backtrace) from [<c03115e9>] (show_stack+0x11/0x14)
[<c03115e9>] (show_stack) from [<c0519f51>] (dump_stack+0x81/0xa8)
[<c0519f51>] (dump_stack) from [<c032185d>] (warn_slowpath_common+0x69/0x90)
[<c032185d>] (warn_slowpath_common) from [<c03218f3>] 
(warn_slowpath_null+0x17/0x1c)
[<c03218f3>] (warn_slowpath_null) from [<c038e87d>] (ftrace_bug+0x1b1/0x234)
[<c038e87d>] (ftrace_bug) from [<c038ebd5>] (ftrace_process_locs+0x285/0x3e8)
[<c038ebd5>] (ftrace_process_locs) from [<c0378d79>] (load_module+0x11e9/0x1a44)
[<c0378d79>] (load_module) from [<c037974d>] (SyS_finit_module+0x59/0x84)
[<c037974d>] (SyS_finit_module) from [<c030e981>] (ret_fast_syscall+0x1/0x18)
---[ end trace e1b64ced7a89adcd ]---
ftrace failed to modify [<e9ef7006>] 0xe9ef7006
actual: 02:f0:3b:fa
ftrace record flags: 0
(0) expected tramp: c0314265

Signed-off-by: Alexander Sverdlin <alexander.sverd...@nokia.com>
---
 arch/arm/include/asm/ftrace.h |  3 +++
 arch/arm/include/asm/module.h |  1 +
 arch/arm/kernel/ftrace.c      | 62 ++++++++++++++++++++++++++++++++++++-------
 arch/arm/kernel/module-plts.c | 47 +++++++++++++++++++++++++++++---
 4 files changed, 100 insertions(+), 13 deletions(-)

diff --git a/arch/arm/include/asm/ftrace.h b/arch/arm/include/asm/ftrace.h
index 9e842ff..faeb6b1 100644
--- a/arch/arm/include/asm/ftrace.h
+++ b/arch/arm/include/asm/ftrace.h
@@ -19,6 +19,9 @@ struct dyn_arch_ftrace {
 #ifdef CONFIG_OLD_MCOUNT
        bool    old_mcount;
 #endif
+#ifdef CONFIG_ARM_MODULE_PLTS
+       struct module *mod;
+#endif
 };
 
 static inline unsigned long ftrace_call_adjust(unsigned long addr)
diff --git a/arch/arm/include/asm/module.h b/arch/arm/include/asm/module.h
index 6996405..e3d7a51 100644
--- a/arch/arm/include/asm/module.h
+++ b/arch/arm/include/asm/module.h
@@ -30,6 +30,7 @@ struct plt_entries {
 
 struct mod_plt_sec {
        struct elf32_shdr       *plt;
+       struct plt_entries      *plt_ent;
        int                     plt_count;
 };
 
diff --git a/arch/arm/kernel/ftrace.c b/arch/arm/kernel/ftrace.c
index 5617932..b55355f 100644
--- a/arch/arm/kernel/ftrace.c
+++ b/arch/arm/kernel/ftrace.c
@@ -98,6 +98,19 @@ int ftrace_arch_code_modify_post_process(void)
 
 static unsigned long ftrace_call_replace(unsigned long pc, unsigned long addr)
 {
+       s32 offset = addr - pc;
+       s32 blim = 0xfe000008;
+       s32 flim = 0x02000004;
+
+       if (IS_ENABLED(CONFIG_THUMB2_KERNEL)) {
+               blim = 0xff000004;
+               flim = 0x01000002;
+       }
+
+       if (IS_ENABLED(CONFIG_ARM_MODULE_PLTS) &&
+           (offset < blim || offset > flim))
+               return 0;
+
        return arm_gen_branch_link(pc, addr);
 }
 
@@ -166,10 +179,22 @@ int ftrace_make_call(struct dyn_ftrace *rec, unsigned 
long addr)
 {
        unsigned long new, old;
        unsigned long ip = rec->ip;
+       unsigned long aaddr = adjust_address(rec, addr);
 
        old = ftrace_nop_replace(rec);
 
-       new = ftrace_call_replace(ip, adjust_address(rec, addr));
+       new = ftrace_call_replace(ip, aaddr);
+
+#ifdef CONFIG_ARM_MODULE_PLTS
+       if (!new) {
+               struct module *mod = rec->arch.mod;
+
+               if (mod) {
+                       aaddr = get_module_plt(mod, ip, aaddr);
+                       new = ftrace_call_replace(ip, aaddr);
+               }
+       }
+#endif
 
        return ftrace_modify_code(rec->ip, old, new, true);
 }
@@ -199,20 +224,39 @@ int ftrace_make_nop(struct module *mod,
        unsigned long new;
        int ret;
 
-       old = ftrace_call_replace(ip, adjust_address(rec, addr));
-       new = ftrace_nop_replace(rec);
-       ret = ftrace_modify_code(ip, old, new, true);
+#ifdef CONFIG_ARM_MODULE_PLTS
+       /* mod is only supplied during module loading */
+       if (!mod)
+               mod = rec->arch.mod;
+       else
+               rec->arch.mod = mod;
+#endif
 
-#ifdef CONFIG_OLD_MCOUNT
-       if (ret == -EINVAL && addr == MCOUNT_ADDR) {
-               rec->arch.old_mcount = true;
+       for (;;) {
+               unsigned long aaddr = adjust_address(rec, addr);
+
+               old = ftrace_call_replace(ip, aaddr);
+
+#ifdef CONFIG_ARM_MODULE_PLTS
+               if (!old && mod) {
+                       aaddr = get_module_plt(mod, ip, aaddr);
+                       old = ftrace_call_replace(ip, aaddr);
+               }
+#endif
 
-               old = ftrace_call_replace(ip, adjust_address(rec, addr));
                new = ftrace_nop_replace(rec);
                ret = ftrace_modify_code(ip, old, new, true);
-       }
+
+#ifdef CONFIG_OLD_MCOUNT
+               if (ret == -EINVAL && !rec->arch.old_mcount) {
+                       rec->arch.old_mcount = true;
+                       continue;
+               }
 #endif
 
+               break;
+       }
+
        return ret;
 }
 
diff --git a/arch/arm/kernel/module-plts.c b/arch/arm/kernel/module-plts.c
index f272711..0951270 100644
--- a/arch/arm/kernel/module-plts.c
+++ b/arch/arm/kernel/module-plts.c
@@ -7,6 +7,7 @@
  */
 
 #include <linux/elf.h>
+#include <linux/ftrace.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/sort.h>
@@ -22,18 +23,54 @@
                                                    (PLT_ENT_STRIDE - 8))
 #endif
 
+static const u32 fixed_plts[] = {
+#ifdef CONFIG_FUNCTION_TRACER
+       FTRACE_ADDR,
+       MCOUNT_ADDR,
+#ifdef CONFIG_OLD_MCOUNT
+       (unsigned long)ftrace_caller_old,
+       (unsigned long)mcount,
+#endif
+#endif
+};
+
 static bool in_init(const struct module *mod, unsigned long loc)
 {
        return loc - (u32)mod->init_layout.base < mod->init_layout.size;
 }
 
+static void prealloc_fixed(struct mod_plt_sec *pltsec, struct plt_entries *plt)
+{
+       int i;
+
+       if (!ARRAY_SIZE(fixed_plts))
+               return;
+
+       for (i = 0; i < ARRAY_SIZE(plt->ldr); ++i)
+               plt->ldr[i] = PLT_ENT_LDR;
+       memcpy(plt->lit, fixed_plts, sizeof(fixed_plts));
+       pltsec->plt_count = ARRAY_SIZE(fixed_plts);
+}
+
 u32 get_module_plt(struct module *mod, unsigned long loc, Elf32_Addr val)
 {
        struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
                                                          &mod->arch.init;
+       struct plt_entries *plt;
+       int idx;
 
-       struct plt_entries *plt = (struct plt_entries *)pltsec->plt->sh_addr;
-       int idx = 0;
+       /* cache the address, ELF header is available only during module load */
+       if (!pltsec->plt_ent)
+               pltsec->plt_ent = (struct plt_entries *)pltsec->plt->sh_addr;
+       plt = pltsec->plt_ent;
+
+       if (!pltsec->plt_count)
+               prealloc_fixed(pltsec, plt);
+
+       idx = ARRAY_SIZE(fixed_plts);
+       while (idx)
+               if (plt->lit[--idx] == val)
+                       return (u32)&plt->ldr[idx];
 
        /*
         * Look for an existing entry pointing to 'val'. Given that the
@@ -182,8 +219,8 @@ static unsigned int count_plts(const Elf32_Sym *syms, 
Elf32_Addr base,
 int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr *sechdrs,
                              char *secstrings, struct module *mod)
 {
-       unsigned long core_plts = 0;
-       unsigned long init_plts = 0;
+       unsigned long core_plts = ARRAY_SIZE(fixed_plts);
+       unsigned long init_plts = ARRAY_SIZE(fixed_plts);
        Elf32_Shdr *s, *sechdrs_end = sechdrs + ehdr->e_shnum;
        Elf32_Sym *syms = NULL;
 
@@ -238,6 +275,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr 
*sechdrs,
        mod->arch.core.plt->sh_size = round_up(core_plts * PLT_ENT_SIZE,
                                               sizeof(struct plt_entries));
        mod->arch.core.plt_count = 0;
+       mod->arch.core.plt_ent = NULL;
 
        mod->arch.init.plt->sh_type = SHT_NOBITS;
        mod->arch.init.plt->sh_flags = SHF_EXECINSTR | SHF_ALLOC;
@@ -245,6 +283,7 @@ int module_frob_arch_sections(Elf_Ehdr *ehdr, Elf_Shdr 
*sechdrs,
        mod->arch.init.plt->sh_size = round_up(init_plts * PLT_ENT_SIZE,
                                               sizeof(struct plt_entries));
        mod->arch.init.plt_count = 0;
+       mod->arch.init.plt_ent = NULL;
 
        pr_debug("%s: plt=%x, init.plt=%x\n", __func__,
                 mod->arch.core.plt->sh_size, mod->arch.init.plt->sh_size);
-- 
2.4.6

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