[PATCH v4 1/4] iio: prepare iio_format_value to handle higher-precision IIO_VAL_DECIMAL64_* formats

[Victor Chernyakin]

Prepare iio_format_value for an upcoming patch that adds formats up
to IIO_VAL_DECIMAL64_YOCTO (a scale of 24 digits) by avoiding
computing 10^scale when doing so would overflow, and by avoiding
undefined behavior due to abs(LLONG_MIN) (which couldn't happen
previously).

Signed-off-by: Victor Chernyakin <chernyakin.victor.j@xxxxxxxxxxx>
---
 drivers/iio/industrialio-core.c | 24 ++++++++++++++++++++----
 include/linux/iio/iio.h         |  3 +++
 2 files changed, 23 insertions(+), 4 deletions(-)

diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 767a7794624a..42e92a79d1ee 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -653,6 +653,16 @@ int iio_read_mount_matrix(struct device *dev, struct iio_mount_matrix *matrix)
 }
 EXPORT_SYMBOL(iio_read_mount_matrix);
 
+/*
+ * Regular abs(LLONG_MIN) causes signed overflow, which is undefined
+ * behavior. This variant avoids that by performing the computation in an
+ * unsigned type, for which negation is always well-defined.
+ */
+static inline u64 abs_s64_no_overflow(s64 x)
+{
+	return x < 0 ? -(u64)x : (u64)x;
+}
+
 static ssize_t __iio_format_value(char *buf, size_t offset, unsigned int type,
 				  int size, const int *vals)
 {
@@ -720,13 +730,19 @@ static ssize_t __iio_format_value(char *buf, size_t offset, unsigned int type,
 		int scale = type - IIO_VAL_DECIMAL64_BASE;
 		s64 frac;
 
-		tmp2 = div64_s64_rem(iio_val_s64_compose(vals[0], vals[1]),
-				     int_pow(10, scale), &frac);
+		if (scale <= IIO_INTLOG10_S64_MAX) {
+			tmp2 = div64_s64_rem(iio_val_s64_compose(vals[0], vals[1]),
+					     int_pow(10, scale), &frac);
+		} else {
+			tmp2 = 0;
+			frac = iio_val_s64_compose(vals[0], vals[1]);
+		}
+
 		if (tmp2 == 0 && frac < 0)
 			l += sysfs_emit_at(buf, offset, "-");
 
-		l += sysfs_emit_at(buf, offset + l, "%lld.%0*lld", tmp2, scale,
-				   abs(frac));
+		l += sysfs_emit_at(buf, offset + l, "%lld.%0*llu", tmp2, scale,
+				   abs_s64_no_overflow(frac));
 		return l;
 	}
 	default:
diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
index 711c00f67371..7cb72ae282b6 100644
--- a/include/linux/iio/iio.h
+++ b/include/linux/iio/iio.h
@@ -1132,4 +1132,7 @@ int iio_str_to_fixpoint(const char *str, int fract_mult, int *integer,
  */
 #define IIO_M_S_2_TO_G(ms2) (((ms2) * 100000ULL + 980665ULL / 2) / 980665ULL)
 
+#define IIO_INTLOG10_S64_MAX 18
+#define IIO_INTLOG10_U64_MAX 19
+
 #endif /* _INDUSTRIAL_IO_H_ */
-- 
2.53.0