diff --git a/include/util.hpp b/include/util.hpp
index 61232bd6..1ec2c49d 100644
--- a/include/util.hpp
+++ b/include/util.hpp
@@ -26,6 +26,9 @@ enum NumberBase {
 int xfclose(FILE *file);
 int xclose(int fd);
 
+// Measure file size with `fseek` and `ftell` idiom
+long seekSize(FILE *file);
+
 // Locale-independent character class functions
 bool isNewline(int c);
 bool isBlankSpace(int c);
diff --git a/src/asm/actions.cpp b/src/asm/actions.cpp
index a3e22308..de1ba1ce 100644
--- a/src/asm/actions.cpp
+++ b/src/asm/actions.cpp
@@ -18,7 +18,7 @@
 #include "extern/utf8decoder.hpp"
 #include "helpers.hpp"
 #include "linkdefs.hpp"
-#include "util.hpp" // xfclose
+#include "util.hpp" // xfclose, seekSize
 
 #include "asm/charmap.hpp"
 #include "asm/format.hpp"
@@ -151,13 +151,12 @@ std::optional<std::string> act_ReadFile(std::string const &name, uint32_t maxLen
 	Defer closeFile{[&] { xfclose(file); }};
 
 	size_t readSize = maxLen;
-	if (fseek(file, 0, SEEK_END) == 0) {
+	if (long fileSize = seekSize(file); fileSize != -1) {
 		// If the file is seekable and shorter than the max length,
 		// just read as many bytes as there are
-		if (long fileSize = ftell(file); static_cast<size_t>(fileSize) < readSize) {
+		if (static_cast<size_t>(fileSize) < readSize) {
 			readSize = fileSize;
 		}
-		fseek(file, 0, SEEK_SET);
 		// LCOV_EXCL_START
 	} else if (errno != ESPIPE) {
 		error(
diff --git a/src/asm/section.cpp b/src/asm/section.cpp
index 63d38f22..165b5fb8 100644
--- a/src/asm/section.cpp
+++ b/src/asm/section.cpp
@@ -21,7 +21,7 @@
 #include "helpers.hpp"
 #include "itertools.hpp" // InsertionOrderedMap
 #include "linkdefs.hpp"
-#include "util.hpp" // xfclose
+#include "util.hpp" // xfclose, seekSize
 
 #include "asm/fstack.hpp"
 #include "asm/lexer.hpp"
@@ -977,14 +977,13 @@ bool sect_BinaryFile(std::string const &name, uint32_t startPos) {
 	}
 	Defer closeFile{[&] { xfclose(file); }};
 
-	if (fseek(file, 0, SEEK_END) == 0) {
-		if (unsigned long fsize = ftell(file);
-		    startPos > fsize) { // `ftell` cannot fail here, since `fseek` succeeded.
+	if (long fileSize = seekSize(file); fileSize != -1) {
+		if (startPos > static_cast<size_t>(fileSize)) {
 			error(
-			    "Specified start position (%" PRIu32 ") is greater than length of \"%s\" (%lu)",
+			    "Specified start position (%" PRIu32 ") is greater than length of \"%s\" (%ld)",
 			    startPos,
 			    name.c_str(),
-			    fsize
+			    fileSize
 			);
 			return false;
 		}
@@ -1038,24 +1037,23 @@ bool sect_BinaryFileSlice(std::string const &name, uint32_t startPos, uint32_t l
 	}
 	Defer closeFile{[&] { xfclose(file); }};
 
-	if (fseek(file, 0, SEEK_END) == 0) {
-		if (unsigned long fsize = ftell(file);
-		    startPos > fsize) { // `ftell` cannot fail here, since `fseek` succeeded.
+	if (long fileSize = seekSize(file); fileSize != -1) {
+		if (startPos > static_cast<size_t>(fileSize)) {
 			error(
-			    "Specified start position (%" PRIu32 ") is greater than length of \"%s\" (%lu)",
+			    "Specified start position (%" PRIu32 ") is greater than length of \"%s\" (%ld)",
 			    startPos,
 			    name.c_str(),
-			    fsize
+			    fileSize
 			);
 			return false;
-		} else if (startPos + length > fsize) {
+		} else if (startPos + length > static_cast<size_t>(fileSize)) {
 			error(
 			    "Specified range in `INCBIN` file \"%s\" is out of bounds (%" PRIu32 " + %" PRIu32
 			    " > %ld)",
 			    name.c_str(),
 			    startPos,
 			    length,
-			    fsize
+			    fileSize
 			);
 			return false;
 		}
diff --git a/src/link/output.cpp b/src/link/output.cpp
index 587b2b48..6e273367 100644
--- a/src/link/output.cpp
+++ b/src/link/output.cpp
@@ -112,16 +112,13 @@ static uint32_t checkOverlaySize() {
 		return 0;
 	}
 
-	if (fseek(overlayFile, 0, SEEK_END) != 0) {
+	long overlaySize = seekSize(overlayFile);
+
+	if (overlaySize == -1) {
 		warnx("Overlay file is not seekable, cannot check if properly formed");
 		return 0;
 	}
 
-	long overlaySize = ftell(overlayFile);
-
-	// Reset back to beginning
-	fseek(overlayFile, 0, SEEK_SET);
-
 	if (overlaySize % BANK_SIZE) {
 		warnx("Overlay file does not have a size multiple of 0x4000");
 	} else if (options.is32kMode && overlaySize != 0x8000) {
diff --git a/src/util.cpp b/src/util.cpp
index 0b97cc18..27bd3490 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -25,6 +25,20 @@ int xclose(int fd) {
 	return close(fd);
 }
 
+long seekSize(FILE *file) {
+	if (fseek(file, 0, SEEK_END) != 0) {
+		return -1;
+	}
+	long size = ftell(file);
+	if (size < 0) {
+		return -1;
+	}
+	if (fseek(file, 0, SEEK_SET) != 0) {
+		return -1;
+	}
+	return size;
+}
+
 bool isNewline(int c) {
 	return c == '\r' || c == '\n';
 }