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Subject[PATCH v6 21/27] x86/ftrace: Adapt function tracing for PIE support
Date
When using PIE with function tracing, the compiler generates a
call through the GOT (call *__fentry__@GOTPCREL). This instruction
takes 6-bytes instead of 5-bytes with a relative call.

If PIE is enabled, replace the 6th byte of the GOT call by a 1-byte nop
so ftrace can handle the previous 5-bytes as before.

Position Independent Executable (PIE) support will allow to extend the
KASLR randomization range below 0xffffffff80000000.

Signed-off-by: Thomas Garnier <thgarnie@chromium.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
arch/x86/kernel/ftrace.c | 51 ++++++++++++++++++++++++--
scripts/recordmcount.c | 78 ++++++++++++++++++++++++++--------------
2 files changed, 101 insertions(+), 28 deletions(-)

diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c
index 8257a59704ae..82feb8c7a47e 100644
--- a/arch/x86/kernel/ftrace.c
+++ b/arch/x86/kernel/ftrace.c
@@ -102,7 +102,7 @@ static const unsigned char *ftrace_nop_replace(void)

static int
ftrace_modify_code_direct(unsigned long ip, unsigned const char *old_code,
- unsigned const char *new_code)
+ unsigned const char *new_code)
{
unsigned char replaced[MCOUNT_INSN_SIZE];

@@ -135,6 +135,53 @@ ftrace_modify_code_direct(unsigned long ip, unsigned const char *old_code,
return 0;
}

+/* Bytes before call GOT offset */
+static const unsigned char got_call_preinsn[] = { 0xff, 0x15 };
+
+static int
+ftrace_modify_initial_code(unsigned long ip, unsigned const char *old_code,
+ unsigned const char *new_code)
+{
+ unsigned char replaced[MCOUNT_INSN_SIZE + 1];
+
+ /*
+ * If PIE is not enabled default to the original approach to code
+ * modification.
+ */
+ if (!IS_ENABLED(CONFIG_X86_PIE))
+ return ftrace_modify_code_direct(ip, old_code, new_code);
+
+ ftrace_expected = old_code;
+
+ /* Ensure the instructions point to a call to the GOT */
+ if (probe_kernel_read(replaced, (void *)ip, sizeof(replaced))) {
+ WARN_ONCE(1, "invalid function");
+ return -EFAULT;
+ }
+
+ if (memcmp(replaced, got_call_preinsn, sizeof(got_call_preinsn))) {
+ WARN_ONCE(1, "invalid function call");
+ return -EINVAL;
+ }
+
+ /*
+ * Build a nop slide with a 5-byte nop and 1-byte nop to keep the ftrace
+ * hooking algorithm working with the expected 5 bytes instruction.
+ */
+ memset(replaced, ideal_nops[1][0], sizeof(replaced));
+ memcpy(replaced, new_code, MCOUNT_INSN_SIZE);
+
+ ip = text_ip_addr(ip);
+
+ if (probe_kernel_write((void *)ip, replaced, sizeof(replaced)))
+ return -EPERM;
+
+ sync_core();
+
+ return 0;
+
+}
+
int ftrace_make_nop(struct module *mod,
struct dyn_ftrace *rec, unsigned long addr)
{
@@ -153,7 +200,7 @@ int ftrace_make_nop(struct module *mod,
* just modify the code directly.
*/
if (addr == MCOUNT_ADDR)
- return ftrace_modify_code_direct(rec->ip, old, new);
+ return ftrace_modify_initial_code(rec->ip, old, new);

ftrace_expected = NULL;

diff --git a/scripts/recordmcount.c b/scripts/recordmcount.c
index a50a2aa963ad..4b8bd746ed2e 100644
--- a/scripts/recordmcount.c
+++ b/scripts/recordmcount.c
@@ -171,33 +171,9 @@ umalloc(size_t size)
return addr;
}

-static unsigned char ideal_nop5_x86_64[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 };
-static unsigned char ideal_nop5_x86_32[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 };
-static unsigned char *ideal_nop;
-
static char rel_type_nop;
-
static int (*make_nop)(void *map, size_t const offset);
-
-static int make_nop_x86(void *map, size_t const offset)
-{
- uint32_t *ptr;
- unsigned char *op;
-
- /* Confirm we have 0xe8 0x0 0x0 0x0 0x0 */
- ptr = map + offset;
- if (*ptr != 0)
- return -1;
-
- op = map + offset - 1;
- if (*op != 0xe8)
- return -1;
-
- /* convert to nop */
- ulseek(fd_map, offset - 1, SEEK_SET);
- uwrite(fd_map, ideal_nop, 5);
- return 0;
-}
+static unsigned char *ideal_nop;

static unsigned char ideal_nop4_arm_le[4] = { 0x00, 0x00, 0xa0, 0xe1 }; /* mov r0, r0 */
static unsigned char ideal_nop4_arm_be[4] = { 0xe1, 0xa0, 0x00, 0x00 }; /* mov r0, r0 */
@@ -447,6 +423,49 @@ static void MIPS64_r_info(Elf64_Rel *const rp, unsigned sym, unsigned type)
}).r_info;
}

+static unsigned char ideal_nop5_x86_64[5] = { 0x0f, 0x1f, 0x44, 0x00, 0x00 };
+static unsigned char ideal_nop6_x86_64[6] = { 0x66, 0x0f, 0x1f, 0x44, 0x00, 0x00 };
+static unsigned char ideal_nop5_x86_32[5] = { 0x3e, 0x8d, 0x74, 0x26, 0x00 };
+static size_t ideal_nop_x86_size;
+
+static unsigned char stub_default_x86[2] = { 0xe8, 0x00 }; /* call relative */
+static unsigned char stub_got_x86[3] = { 0xff, 0x15, 0x00 }; /* call .got */
+static unsigned char *stub_x86;
+static size_t stub_x86_size;
+
+static int make_nop_x86(void *map, size_t const offset)
+{
+ uint32_t *ptr;
+ size_t stub_offset = offset - stub_x86_size;
+
+ /* confirm we have the expected stub */
+ ptr = map + stub_offset;
+ if (memcmp(ptr, stub_x86, stub_x86_size))
+ return -1;
+
+ /* convert to nop */
+ ulseek(fd_map, stub_offset, SEEK_SET);
+ uwrite(fd_map, ideal_nop, ideal_nop_x86_size);
+ return 0;
+}
+
+/* Swap the stub and nop for a got call if the binary is built with PIE */
+static int is_fake_mcount_x86_x64(Elf64_Rel const *rp)
+{
+ if (ELF64_R_TYPE(rp->r_info) == R_X86_64_GOTPCREL) {
+ ideal_nop = ideal_nop6_x86_64;
+ ideal_nop_x86_size = sizeof(ideal_nop6_x86_64);
+ stub_x86 = stub_got_x86;
+ stub_x86_size = sizeof(stub_got_x86);
+ mcount_adjust_64 = 1 - stub_x86_size;
+ }
+
+ /* Once the relocation was checked, rollback to default */
+ is_fake_mcount64 = fn_is_fake_mcount64;
+ return is_fake_mcount64(rp);
+}
+
+
static void
do_file(char const *const fname)
{
@@ -509,6 +528,9 @@ do_file(char const *const fname)
rel_type_nop = R_386_NONE;
make_nop = make_nop_x86;
ideal_nop = ideal_nop5_x86_32;
+ ideal_nop_x86_size = sizeof(ideal_nop5_x86_32);
+ stub_x86 = stub_default_x86;
+ stub_x86_size = sizeof(stub_default_x86);
mcount_adjust_32 = -1;
break;
case EM_ARM: reltype = R_ARM_ABS32;
@@ -533,9 +555,13 @@ do_file(char const *const fname)
case EM_X86_64:
make_nop = make_nop_x86;
ideal_nop = ideal_nop5_x86_64;
+ ideal_nop_x86_size = sizeof(ideal_nop5_x86_64);
+ stub_x86 = stub_default_x86;
+ stub_x86_size = sizeof(stub_default_x86);
reltype = R_X86_64_64;
rel_type_nop = R_X86_64_NONE;
- mcount_adjust_64 = -1;
+ is_fake_mcount64 = is_fake_mcount_x86_x64;
+ mcount_adjust_64 = 1 - stub_x86_size;
break;
} /* end switch */

--
2.20.1.495.gaa96b0ce6b-goog
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