mirrors/cosmopolitan
1
0
Fork 0
mirror of https://github.com/jart/cosmopolitan.git synced 2025-05-29 00:32:29 +00:00
Code Issues Projects Releases Packages Wiki Activity Actions 1

Fix UB in gdtoa hexadecimal float scanf and strtod (#1288)

Browse source 
When reading hexadecimal floats, cosmopolitan would previously sometimes
print a number of warnings relating to undefined behavior on left shift:

third_party/gdtoa/gethex.c:172: ubsan warning: signed left shift changed
sign bit or overflowed 12 'int' 28 'int' is undefined behavior

This is because gdtoa assumes left shifts are safe when overflow happens
even on signed integers - this is false: the C standard considers it UB.
This is easy to fix, by simply casting the shifted value to unsigned, as
doing so does not change the value or the semantics of the left shifting
(except for avoiding the undefined behavior, as the C standard specifies
that unsigned overflow yields wraparound, avoiding undefined behaviour).

This commit does this, and adds a testcase that previously triggered UB.
(this also adds test macros to test for exact float equality, instead of
the existing {EXPECT,ASSERT}_FLOAT_EQ macros which only tests inputs for
being "almost equal" (with a significant epsilon) whereas exact equality
makes more sense for certain things such as reading floats from strings,
and modifies other testcases for sscanf/fscanf of floats to utilize it).
This commit is contained in:
Gabriel Ravier 2024-09-15 02:11:04 +02:00 • committed by GitHub
parent 7f21547122
commit e3d28de8a6
No known key found for this signature in database
GPG key ID: B5690EEEBB952194
6 changed files with 88 additions and 14 deletions
Download patch file Download diff file Expand all files Collapse all files

2
test/libc/stdio/fscanf_test.c
View file

@ -27,7 +27,7 @@ TEST(fscanf, test_readAfterFloat) {
EXPECT_EQ(4, fscanf(f, "%f%x%f%x", &f1, &i1, &f2, &i2));
EXPECT_TRUE(isinf(f1));
EXPECT_EQ(0xDEAD, i1);
EXPECT_EQ(-0.125e-2f, f2);
EXPECT_FLOAT_EXACTLY_EQ(-0.125e-2f, f2);
EXPECT_EQ(0xBEEF, i2);
fclose(f);
}

30
test/libc/stdio/sscanf_test.c
View file

@ -338,17 +338,17 @@ TEST(sscanf, flexdecimal_hex) {
TEST(sscanf, floating_point_simple) {
float x = 666.666f, y = x, z = y;
EXPECT_EQ(3, sscanf("0.3715 .3715 3715", "%f %f %f", &x, &y, &z));
EXPECT_EQ(0.3715f, x);
EXPECT_EQ(0.3715f, y);
EXPECT_EQ(3715.0f, z);
EXPECT_FLOAT_EXACTLY_EQ(0.3715f, x);
EXPECT_FLOAT_EXACTLY_EQ(0.3715f, y);
EXPECT_FLOAT_EXACTLY_EQ(3715.0f, z);
}
TEST(sscanf, floating_point_simple_double_precision) {
double x = 666.666, y = x, z = y;
EXPECT_EQ(3, sscanf("0.3715 .3715 3715", "%lf %lf %lf", &x, &y, &z));
EXPECT_EQ(0.3715, x);
EXPECT_EQ(0.3715, y);
EXPECT_EQ(3715.0, z);
EXPECT_DOUBLE_EXACTLY_EQ(0.3715, x);
EXPECT_DOUBLE_EXACTLY_EQ(0.3715, y);
EXPECT_DOUBLE_EXACTLY_EQ(3715.0, z);
}
TEST(sscanf, floating_point_nan) {
@ -426,12 +426,12 @@ TEST(sscanf, floating_point_documentation_examples) {
2, sscanf("0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz -0.0000000123junk junk",
"%f %f %f %f %f", &f, &g, &h, &i, &j));
EXPECT_EQ(111.11f, a);
EXPECT_EQ(-2.22f, b);
EXPECT_FLOAT_EXACTLY_EQ(111.11f, a);
EXPECT_FLOAT_EXACTLY_EQ(-2.22f, b);
EXPECT_TRUE(isnan(c));
EXPECT_TRUE(isnan(d));
EXPECT_TRUE(isinf(e));
EXPECT_EQ(0X1.BC70A3D70A3D7P+6f, f);
EXPECT_FLOAT_EXACTLY_EQ(0X1.BC70A3D70A3D7P+6f, f);
EXPECT_TRUE(isinf(g));
}
@ -445,12 +445,12 @@ TEST(sscanf, floating_point_documentation_examples_double_precision) {
2, sscanf("0X1.BC70A3D70A3D7P+6 1.18973e+4932zzz -0.0000000123junk junk",
"%lf %lf %lf %lf %lf", &f, &g, &h, &i, &j));
EXPECT_EQ(111.11, a);
EXPECT_EQ(-2.22, b);
EXPECT_DOUBLE_EXACTLY_EQ(111.11, a);
EXPECT_DOUBLE_EXACTLY_EQ(-2.22, b);
EXPECT_TRUE(isnan(c));
EXPECT_TRUE(isnan(d));
EXPECT_TRUE(isinf(e));
EXPECT_EQ(0X1.BC70A3D70A3D7P+6, f);
EXPECT_DOUBLE_EXACTLY_EQ(0X1.BC70A3D70A3D7P+6, f);
EXPECT_TRUE(isinf(g));
}
@ -506,3 +506,9 @@ TEST(scanf, n) {
ASSERT_EQ(1848, port);
ASSERT_EQ(12, len);
}
TEST(sscanf, floating_point_hexadecimal) {
double a = 0;
ASSERT_EQ(1, sscanf("0x1.5014c3472bc2c0000000p-123", "%lf", &a));
ASSERT_DOUBLE_EXACTLY_EQ(0x1.5014c3472bc2c0000000p-123, a);
}