scripts/gdb: make lx-dmesg command work (reliably)

PATCH Superseded André Draszik 4 participants 8 messages May 26, 2017 – May 31, 2017 GDB kernel debugging helper scripts

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André Draszik

author

2017-05-26 11:05 UTC #0 lore raw

PATCH 1/1 scripts/gdb: make lx-dmesg command work (reliably)

lx-dmesg needs access to the log_buf symbol from printk.c. Unfortunately, the symbol log_buf also exists in BPF's verifier.c and hence gdb can pick one or the other. If it happens to pick BPF's log_buf, lx-dmesg doesn't work:

(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0

Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html

(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":

File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;

File <linux.git>/kernel/printk/printk.c:
static char *log_buf;
(gdb) p 'verifier.c'::log_buf
$1 = 0x0
(gdb) p 'printk.c'::log_buf
$2 = 0x811a6aa0 <__log_buf> ""

By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c

Signed-off-by André Draszik <[email protected]>

---
 scripts/gdb/linux/dmesg.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

scripts/gdb/linux/dmesg.py +4 -4

index f9b92ece7834..11f2397d40ed 100644
@@ -23,10 +23,10 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval("'printk.c'::log_buf")).split()[0], 16)
+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
 
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx

Signature · 1 line

2.11.0

Jan Kiszka

maintainer

2017-05-26 11:13 UTC #1 lore raw

Quoted 41 lines · André Draszik

lx-dmesg needs access to the log_buf symbol from printk.c. Unfortunately, the symbol log_buf also exists in BPF's verifier.c and hence gdb can pick one or the other. If it happens to pick BPF's log_buf, lx-dmesg doesn't work:
(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0
Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html
(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":
File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;
File <linux.git>/kernel/printk/printk.c:
static char *log_buf;
(gdb) p 'verifier.c'::log_buf
$1 = 0x0
(gdb) p 'printk.c'::log_buf
$2 = 0x811a6aa0 <__log_buf> ""
By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c
Signed-off-by André Draszik <[email protected]>
---
 scripts/gdb/linux/dmesg.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)
scripts/gdb/linux/dmesg.py +0 -0
index f9b92ece7834..11f2397d40ed 100644

scripts/gdb/linux/dmesg.py class LxDmesg(gdb.Command): · line 26

Quoted 8 lines

scripts/gdb/linux/dmesg.py +1 -4

@@ -23,10 +23,10 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval("'printk.c'::log_buf")).split()[0], 16)

Overlong line. Please stay pep8 compliant.

scripts/gdb/linux/dmesg.py class LxDmesg(gdb.Command): · line 32

Quoted 7 lines

scripts/gdb/linux/dmesg.py +3 -0

+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
 
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx

Looks good otherwise, thanks for addressing this!

Jan

Signature · 2 lines

Siemens AG, Corporate Technology, CT RDA ITP SES-DE
Corporate Competence Center Embedded Linux

Andy Shevchenko

2017-05-27 14:01 UTC #2 lore raw

Quoted 5 lines · Jan Kiszka

Quoted 1 line · André Draszik
+ log_buf_addr = int(str(gdb.parse_and_eval("'printk.c'::log_buf")).split()[0], 16)
Overlong line. Please stay pep8 compliant.

What I heard from colleague of mine couple of years ago, they (their team) moved to 100 characters in Python scripts because of somehow stupid recommendations from last century.

Signature · 2 lines

With Best Regards,
Andy Shevchenko

Jan Kiszka

maintainer

2017-05-28 16:30 UTC #3 lore raw

Quoted 10 lines · Andy Shevchenko

Quoted 5 lines · Jan Kiszka
Quoted 1 line · André Draszik
+ log_buf_addr = int(str(gdb.parse_and_eval("'printk.c'::log_buf")).split()[0], 16)
Overlong line. Please stay pep8 compliant.
What I heard from colleague of mine couple of years ago, they (their team) moved to 100 characters in Python scripts because of somehow stupid recommendations from last century.

Well, it happens to be in line with the kernel preference for C code. We can discuss a different style when the kernel relaxes its own. ;)

Jan

Signature · 2 lines

Siemens AG, Corporate Technology, CT RDA ITP SES-DE
Corporate Competence Center Embedded Linux

André Draszik

author

2017-05-26 11:14 UTC #4 lore raw

PATCH v2 1/1 scripts/gdb: make lx-dmesg command work (reliably)

In reply to André Draszik

(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0

Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html

(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":

File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;

File <linux.git>/kernel/printk/printk.c:
static char *log_buf;
(gdb) p 'verifier.c'::log_buf
$1 = 0x0
(gdb) p 'printk.c'::log_buf
$2 = 0x811a6aa0 <__log_buf> ""
(gdb) p &log_buf
$3 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'verifier.c'::log_buf
$4 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'printk.c'::log_buf
$5 = (char **) 0x8048b7d0 <log_buf>

By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c

Signed-off-by André Draszik <[email protected]>

---
v2: Commit message slightly updated

---
 scripts/gdb/linux/dmesg.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

scripts/gdb/linux/dmesg.py +4 -4

index f9b92ece7834..11f2397d40ed 100644
@@ -23,10 +23,10 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval("'printk.c'::log_buf")).split()[0], 16)
+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
 
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx

Signature · 1 line

2.11.0

André Draszik

author

2017-05-26 11:23 UTC #5 lore raw

PATCH v3 1/1 scripts/gdb: make lx-dmesg command work (reliably)

In reply to André Draszik

(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0

Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html

(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":

File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;

By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c

Signed-off-by André Draszik <[email protected]>

---
v3: pep8 compliant
v2: Commit message slightly updated

---
 scripts/gdb/linux/dmesg.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

scripts/gdb/linux/dmesg.py +5 -4

index f9b92ece7834..5afd1098e33a 100644
@@ -23,10 +23,11 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval(
+            "'printk.c'::log_buf")).split()[0], 16)
+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
 
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx

Signature · 1 line

2.11.0

Jan Kiszka

maintainer Acked-by on v3 1/1

2017-05-26 11:30 UTC #6 lore raw

Quoted 57 lines · André Draszik

lx-dmesg needs access to the log_buf symbol from printk.c. Unfortunately, the symbol log_buf also exists in BPF's verifier.c and hence gdb can pick one or the other. If it happens to pick BPF's log_buf, lx-dmesg doesn't work:
(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0
Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html
(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":
File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;
File <linux.git>/kernel/printk/printk.c:
static char *log_buf;
(gdb) p 'verifier.c'::log_buf
$1 = 0x0
(gdb) p 'printk.c'::log_buf
$2 = 0x811a6aa0 <__log_buf> ""
(gdb) p &log_buf
$3 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'verifier.c'::log_buf
$4 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'printk.c'::log_buf
$5 = (char **) 0x8048b7d0 <log_buf>
By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c
Signed-off-by André Draszik <[email protected]>
---
v3: pep8 compliant
v2: Commit message slightly updated
---
 scripts/gdb/linux/dmesg.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)
scripts/gdb/linux/dmesg.py +0 -2
index f9b92ece7834..5afd1098e33a 100644
@@ -23,10 +23,11 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))

scripts/gdb/linux/dmesg.py class LxDmesg(gdb.Command): · line 33

Quoted 10 lines

scripts/gdb/linux/dmesg.py +5 -2

-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval(
+            "'printk.c'::log_buf")).split()[0], 16)
+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))
 
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx

Acked-by Jan Kiszka <[email protected]>

Andrew, please include in your queue.

Jan

Signature · 2 lines

Siemens AG, Corporate Technology, CT RDA ITP SES-DE
Corporate Competence Center Embedded Linux

Kieran Bingham

Tested-by

2017-05-31 05:10 UTC #7 lore raw

Hi André, Jan, Andrew,

Quoted 34 lines · Jan Kiszka

Quoted 33 lines · André Draszik
lx-dmesg needs access to the log_buf symbol from printk.c. Unfortunately, the symbol log_buf also exists in BPF's verifier.c and hence gdb can pick one or the other. If it happens to pick BPF's log_buf, lx-dmesg doesn't work:
(gdb) lx-dmesg
Python Exception <class 'gdb.MemoryError'> Cannot access memory at address 0x0:
Error occurred in Python command: Cannot access memory at address 0x0 (gdb) p log_buf
$15 = 0x0
Luckily, GDB has a way to deal with this, see
https://sourceware.org/gdb/onlinedocs/gdb/Symbols.html
(gdb) info variables ^log_buf$
All variables matching regular expression "^log_buf$":
File <linux.git>/kernel/bpf/verifier.c:
static char *log_buf;
File <linux.git>/kernel/printk/printk.c:
static char *log_buf;
(gdb) p 'verifier.c'::log_buf
$1 = 0x0
(gdb) p 'printk.c'::log_buf
$2 = 0x811a6aa0 <__log_buf> ""
(gdb) p &log_buf
$3 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'verifier.c'::log_buf
$4 = (char **) 0x8120fe40 <log_buf>
(gdb) p &'printk.c'::log_buf
$5 = (char **) 0x8048b7d0 <log_buf>

Quoted 5 lines

Quoted 5 lines
By being explicit about the location of the symbol, we can make lx-dmesg work again. While at it, do the same for the other symbols we need from printk.c
Signed-off-by André Draszik <[email protected]>

Nice to see you again André :D

Quoted 25 lines

---
v3: pep8 compliant
v2: Commit message slightly updated

---
 scripts/gdb/linux/dmesg.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

scripts/gdb/linux/dmesg.py +5 -4

index f9b92ece7834..5afd1098e33a 100644
@@ -23,10 +23,11 @@ class LxDmesg(gdb.Command):
         super(LxDmesg, self).__init__("lx-dmesg", gdb.COMMAND_DATA)
 
     def invoke(self, arg, from_tty):
-        log_buf_addr = int(str(gdb.parse_and_eval("log_buf")).split()[0], 16)
-        log_first_idx = int(gdb.parse_and_eval("log_first_idx"))
-        log_next_idx = int(gdb.parse_and_eval("log_next_idx"))
-        log_buf_len = int(gdb.parse_and_eval("log_buf_len"))
+        log_buf_addr = int(str(gdb.parse_and_eval(
+            "'printk.c'::log_buf")).split()[0], 16)
+        log_first_idx = int(gdb.parse_and_eval("'printk.c'::log_first_idx"))
+        log_next_idx = int(gdb.parse_and_eval("'printk.c'::log_next_idx"))
+        log_buf_len = int(gdb.parse_and_eval("'printk.c'::log_buf_len"))

Quoted 5 lines

scripts/gdb/linux/dmesg.py class LxDmesg(gdb.Command): · line 33
Quoted 3 lines
scripts/gdb/linux/dmesg.py +0 -0
         inf = gdb.inferiors()[0]
         start = log_buf_addr + log_first_idx
Acked-by Jan Kiszka <[email protected]>

Sorry for the delay - I'm in Tokyo for Linux OSS Japan.

This looks good to me and I've verified it locally.

Tested-by Kieran Bingham <[email protected]>

Quoted 5 lines

Andrew, please include in your queue.
Jan

Signature · 3 lines

Regards

Kieran Bingham