[RFC PATCH] Syscall tracing on PPC64_ELF_ABI_V1 without KALLSYMS_ALL

[Michael Jeanson <mjeanson@efficios.com>]

In ad050d2390fccb22aa3e6f65e11757ce7a5a7ca5 we fixed ftrace syscall
tracing on PPC64_ELF_ABI_V1 by looking for the non-dot prefixed symbol
of a syscall.

Ftrace uses kallsyms to locate syscall symbols and those non-dot
prefixed symbols reside in a separate '.opd' section which is not
included by kallsyms.

So we either need to have FTRACE_SYSCALLS select KALLSYMS_ALL on
PPC64_ELF_ABI_V1 or add the '.opd' section symbols to kallsyms.

This patch does the minimum to achieve the latter, it's tested on a
corenet64_smp_defconfig with KALLSYMS_ALL turned off.

I'm unsure which of the alternatives would be better.

---
In 'kernel/module/kallsyms.c' the 'is_core_symbol' function might also
require some tweaking to make all opd symbols available to kallsyms but
that doesn't impact ftrace syscall tracing.

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
---
 include/asm-generic/sections.h | 14 ++++++++++++++
 include/linux/kallsyms.h       |  3 +++
 kernel/kallsyms.c              |  2 ++
 scripts/kallsyms.c             |  1 +
 4 files changed, 20 insertions(+)

diff --git a/include/asm-generic/sections.h b/include/asm-generic/sections.h
index db13bb620f52..1410566957e5 100644
--- a/include/asm-generic/sections.h
+++ b/include/asm-generic/sections.h
@@ -180,6 +180,20 @@ static inline bool is_kernel_rodata(unsigned long addr)
 	       addr < (unsigned long)__end_rodata;
 }
 
+/**
+ * is_kernel_opd - checks if the pointer address is located in the
+ *                 .opd section
+ *
+ * @addr: address to check
+ *
+ * Returns: true if the address is located in .opd, false otherwise.
+ */
+static inline bool is_kernel_opd(unsigned long addr)
+{
+	return addr >= (unsigned long)__start_opd &&
+	       addr < (unsigned long)__end_opd;
+}
+
 /**
  * is_kernel_inittext - checks if the pointer address is located in the
  *                      .init.text section
diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
index 649faac31ddb..9bfb4d8d41a5 100644
--- a/include/linux/kallsyms.h
+++ b/include/linux/kallsyms.h
@@ -43,6 +43,9 @@ static inline int is_ksym_addr(unsigned long addr)
 	if (IS_ENABLED(CONFIG_KALLSYMS_ALL))
 		return is_kernel(addr);
 
+	if (IS_ENABLED(CONFIG_HAVE_FUNCTION_DESCRIPTORS))
+		return is_kernel_text(addr) || is_kernel_inittext(addr) || is_kernel_opd(addr);
+
 	return is_kernel_text(addr) || is_kernel_inittext(addr);
 }
 
diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 60c20f301a6b..009b1ca21618 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -281,6 +281,8 @@ static unsigned long get_symbol_pos(unsigned long addr,
 			symbol_end = (unsigned long)_einittext;
 		else if (IS_ENABLED(CONFIG_KALLSYMS_ALL))
 			symbol_end = (unsigned long)_end;
+		else if (IS_ENABLED(CONFIG_HAVE_FUNCTION_DESCRIPTORS) && is_kernel_opd(addr))
+			symbol_end = (unsigned long)__end_opd;
 		else
 			symbol_end = (unsigned long)_etext;
 	}
diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 03fa07ad45d9..decf31c497f5 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -64,6 +64,7 @@ static unsigned long long relative_base;
 static struct addr_range text_ranges[] = {
 	{ "_stext",     "_etext"     },
 	{ "_sinittext", "_einittext" },
+	{ "__start_opd", "__end_opd" },
 };
 #define text_range_text     (&text_ranges[0])
 #define text_range_inittext (&text_ranges[1])
-- 
2.34.1