[PATCH v3] coccinelle: semantic patch to check for inappropriate do_div() calls

[Wen Yang]

do_div() does a 64-by-32 division.
When the divisor is unsigned long, u64, or s64,
do_div() truncates it to 32 bits, this means it
can test non-zero and be truncated to zero for division.
This semantic patch is inspired by Mateusz Guzik's patch:
commit b0ab99e7736a ("sched: Fix possible divide by zero in avg_atom() calculation")

Signed-off-by: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>
Cc: Julia Lawall <julia.lawall@xxxxxxxx>
Cc: Gilles Muller <Gilles.Muller@xxxxxxx>
Cc: Nicolas Palix <nicolas.palix@xxxxxxx>
Cc: Michal Marek <michal.lkml@xxxxxxxxxxx>
Cc: Matthias Maennich <maennich@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: cocci@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
v3:
- also filter out safe consts for context mode.
- cleanup code.

v2:
- add a special case for constants and checking whether the value is obviously safe and no warning is needed.
- fix 'WARNING:' twice in each case.
- extend the warning to say "consider using div64_xxx instead".

 scripts/coccinelle/misc/do_div.cocci | 155 +++++++++++++++++++++++++++
 1 file changed, 155 insertions(+)
 create mode 100644 scripts/coccinelle/misc/do_div.cocci

diff --git a/scripts/coccinelle/misc/do_div.cocci b/scripts/coccinelle/misc/do_div.cocci
new file mode 100644
index 000000000000..79db083c5208
--- /dev/null
+++ b/scripts/coccinelle/misc/do_div.cocci
@@ -0,0 +1,155 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/// do_div() does a 64-by-32 division.
+/// When the divisor is long, unsigned long, u64, or s64,
+/// do_div() truncates it to 32 bits, this means it can test
+/// non-zero and be truncated to 0 for division on 64bit platforms.
+///
+//# This makes an effort to find those inappropriate do_div() calls.
+//
+// Confidence: Moderate
+// Copyright: (C) 2020 Wen Yang, Alibaba.
+// Comments:
+// Options: --no-includes --include-headers
+
+virtual context
+virtual org
+virtual report
+
+@initialize:python@
+@@
+
+def get_digit_type_and_value(str):
+    is_digit = False
+    value = 0
+
+    try:
+        if (str.isdigit()):
+           is_digit = True
+           value =  int(str, 0)
+        elif (str.upper().endswith('ULL')):
+           is_digit = True
+           value = int(str[:-3], 0)
+        elif (str.upper().endswith('LL')):
+           is_digit = True
+           value = int(str[:-2], 0)
+        elif (str.upper().endswith('UL')):
+           is_digit = True
+           value = int(str[:-2], 0)
+        elif (str.upper().endswith('L')):
+           is_digit = True
+           value = int(str[:-1], 0)
+        elif (str.upper().endswith('U')):
+           is_digit = True
+           value = int(str[:-1], 0)
+    except Exception as e:
+          print('Error:',e)
+          is_digit = False
+          value = 0
+    finally:
+        return is_digit, value
+
+def filter_out_safe_constants(str):
+    is_digit, value = get_digit_type_and_value(str)
+    if (is_digit):
+        if (value >= 0x100000000):
+            return True
+        else:
+            return False
+    else:
+        return True
+
+def construct_warnings(suggested_fun):
+    msg="WARNING: do_div() does a 64-by-32 division, please consider using %s instead."
+    return  msg % suggested_fun
+
+@depends on context@
+expression f;
+long l: script:python() { filter_out_safe_constants(l) };
+unsigned long ul : script:python() { filter_out_safe_constants(ul) };
+u64 ul64 : script:python() { filter_out_safe_constants(ul64) };
+s64 sl64 : script:python() { filter_out_safe_constants(sl64) };
+
+@@
+(
+* do_div(f, l);
+|
+* do_div(f, ul);
+|
+* do_div(f, ul64);
+|
+* do_div(f, sl64);
+)
+
+@r depends on (org || report)@
+expression f;
+position p;
+long l: script:python() { filter_out_safe_constants(l) };
+unsigned long ul : script:python() { filter_out_safe_constants(ul) };
+u64 ul64 : script:python() { filter_out_safe_constants(ul64) };
+s64 sl64 : script:python() { filter_out_safe_constants(sl64) };
+@@
+(
+do_div@p(f, l);
+|
+do_div@p(f, ul);
+|
+do_div@p(f, ul64);
+|
+do_div@p(f, sl64);
+)
+
+@script:python depends on org@
+p << r.p;
+ul << r.ul;
+@@
+
+coccilib.org.print_todo(p[0], construct_warnings("div64_ul"))
+
+@script:python depends on org@
+p << r.p;
+l << r.l;
+@@
+
+coccilib.org.print_todo(p[0], construct_warnings("div64_long"))
+
+@script:python depends on org@
+p << r.p;
+ul64 << r.ul64;
+@@
+
+coccilib.org.print_todo(p[0], construct_warnings("div64_u64"))
+
+@script:python depends on org@
+p << r.p;
+sl64 << r.sl64;
+@@
+
+coccilib.org.print_todo(p[0], construct_warnings("div64_s64"))
+
+@script:python depends on report@
+p << r.p;
+ul << r.ul;
+@@
+
+coccilib.report.print_report(p[0], construct_warnings("div64_ul"))
+
+@script:python depends on report@
+p << r.p;
+l << r.l;
+@@
+
+coccilib.report.print_report(p[0], construct_warnings("div64_long"))
+
+@script:python depends on report@
+p << r.p;
+sl64 << r.sl64;
+@@
+
+coccilib.report.print_report(p[0], construct_warnings("div64_s64"))
+
+@script:python depends on report@
+p << r.p;
+ul64 << r.ul64;
+@@
+
+coccilib.report.print_report(p[0], construct_warnings("div64_u64"))
-- 
2.23.0