libdrgn: add read(2) and pread(2) wrappers that don't return short reads

We have a couple of loops that deal with short reads/EINTR from read(2)
and pread(2), and upcoming changes would need to add more. Add some
wrappers to abstract this away.

drgn_read_memory_file() still needs the loop so it can fault on the
exact offset that returns EIO.

Signed-off-by: Omar Sandoval <osandov@osandov.com>

libdrgn/Doxyfile

@@ -269,7 +269,7 @@ TAB_SIZE               = 4
 # commands \{ and \} for these it is advised to use the version @{ and @} or use
 # a double escape (\\{ and \\})
 
-ALIASES                =
+ALIASES                = manpage{2}="<a href=\"http://man7.org/linux/man-pages/man\2/\1.\2.html\">\1(\2)</a>"
 
 # Set the OPTIMIZE_OUTPUT_FOR_C tag to YES if your project consists of C sources
 # only. Doxygen will then generate output that is more tailored for C. For

libdrgn/Makefile.am

@@ -55,6 +55,8 @@ libdrgnimpl_la_SOURCES = $(ARCH_DEFS_PYS:_defs.py=.c) \
 			 hash_table.c \
 			 hash_table.h \
 			 helpers.h \
+			 io.c \
+			 io.h \
 			 language.c \
 			 language.h \
 			 language_c.c \

libdrgn/io.c

@@ -0,0 +1,50 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+#include <errno.h>
+#include <limits.h>
+#include <unistd.h>
+
+#include "io.h"
+
+ssize_t read_all(int fd, void *buf, size_t count)
+{
+	if (count > SSIZE_MAX) {
+		errno = EINVAL;
+		return -1;
+	}
+	size_t n = 0;
+	while (n < count) {
+		ssize_t r = read(fd, (char *)buf + n, count - n);
+		if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			return r;
+		} else if (r == 0) {
+			break;
+		}
+		n += r;
+	}
+	return n;
+}
+
+ssize_t pread_all(int fd, void *buf, size_t count, off_t offset)
+{
+	if (count > SSIZE_MAX) {
+		errno = EINVAL;
+		return -1;
+	}
+	size_t n = 0;
+	while (n < count) {
+		ssize_t r = pread(fd, (char *)buf + n, count - n, offset + n);
+		if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			return r;
+		} else if (r == 0) {
+			break;
+		}
+		n += r;
+	}
+	return n;
+}

libdrgn/io.h

@@ -0,0 +1,27 @@
+// Copyright (c) Meta Platforms, Inc. and affiliates.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+/**
+ * @file
+ *
+ * Input/output helpers.
+ */
+
+#ifndef DRGN_IO_H
+#define DRGN_IO_H
+
+#include <sys/types.h>
+
+/**
+ * Wrapper around \manpage{read,2} that never returns less bytes than requested unless it
+ * hits end-of-file.
+ */
+ssize_t read_all(int fd, void *buf, size_t count);
+
+/**
+ * Wrapper around \manpage{pread,2} that never returns less bytes than requested unless
+ * it hits end-of-file.
+ */
+ssize_t pread_all(int fd, void *buf, size_t count, off_t offset);
+
+#endif /* DRGN_IO_H */

libdrgn/linux_kernel.c

@@ -23,6 +23,7 @@
 #include "error.h"
 #include "hash_table.h"
 #include "helpers.h"
+#include "io.h"
 #include "linux_kernel.h"
 #include "platform.h"
 #include "program.h"
@@ -630,27 +631,17 @@ kernel_module_iterator_gnu_build_id_live(struct kernel_module_iterator *it,
 			goto out;
 		}
 
-		char *buf = it->build_id_buf;
+		ssize_t r = read_all(fd, it->build_id_buf, st.st_size);
-		size_t size = 0;
-		while (size < st.st_size) {
-			ssize_t r = read(fd, buf + size, st.st_size - size);
 		if (r < 0) {
-				if (errno == EINTR)
+			err = drgn_error_format_os("read", errno, "%s/%s", path,
-					continue;
-				err = drgn_error_format_os("read", errno,
-							   "%s/%s", path,
 						   ent->d_name);
 			close(fd);
 			goto out;
-			} else if (r == 0) {
-				break;
-			}
-			size += r;
 		}
 		close(fd);
 
-		*build_id_len_ret = parse_gnu_build_id_from_note(buf, size,
+		*build_id_len_ret =
-								 false,
+			parse_gnu_build_id_from_note(it->build_id_buf, r, false,
 						     build_id_ret);
 		if (*build_id_len_ret) {
 			err = NULL;

libdrgn/program.c

@@ -21,6 +21,7 @@
 #include "debug_info.h"
 #include "error.h"
 #include "helpers.h"
+#include "io.h"
 #include "language.h"
 #include "linux_kernel.h"
 #include "memory_reader.h"
@@ -205,23 +206,11 @@ static struct drgn_error *has_kdump_signature(const char *path, int fd,
 					      bool *ret)
 {
 	char signature[KDUMP_SIG_LEN];
-	size_t n = 0;
+	ssize_t r = pread_all(fd, signature, sizeof(signature), 0);
-
+	if (r < 0)
-	while (n < sizeof(signature)) {
-		ssize_t sret;
-
-		sret = pread(fd, signature + n, sizeof(signature) - n, n);
-		if (sret == -1) {
-			if (errno == EINTR)
-				continue;
 		return drgn_error_create_os("pread", errno, path);
-		} else if (sret == 0) {
+	*ret = (r == sizeof(signature)
-			*ret = false;
+		&& memcmp(signature, KDUMP_SIGNATURE, sizeof(signature)) == 0);
-			return NULL;
-		}
-		n += sret;
-	}
-	*ret = memcmp(signature, KDUMP_SIGNATURE, sizeof(signature)) == 0;
 	return NULL;
 }