Replace lseek/read into pread for kcore and vmcore reading

Previously crash uses lseek/read for kcore and vmcore reading, this involves
2 syscalls. And we can replace them with pread, only 1 syscall is needed for
kcore/vmcore reading, and we can have a better performance. Please note there
are plenty of places in crash using lseek/read, this patch doesn't modify all
of them, just the most commonly used kcore and diskdump vmcore reading.

Signed-off-by: Tao Liu <ltao@redhat.com>

diskdump.c

@@ -515,16 +515,22 @@ arm_kdump_header_adjust(int header_version)
 static int
 read_pd(int fd, off_t offset, page_desc_t *pd)
 {
-	const off_t failed = (off_t)-1;
+	int ret;
 
 	if (FLAT_FORMAT()) {
 		if (!read_flattened_format(fd, offset, pd, sizeof(*pd)))
 			return READ_ERROR;
 	} else {
-		if (lseek(fd, offset, SEEK_SET) == failed)
+		if (offset < 0) {
+			if (CRASHDEBUG(8))
+				fprintf(fp, "read_pd: invalid offset: %lx\n", offset);
 			return SEEK_ERROR;
-		if (read(fd, pd, sizeof(*pd)) != sizeof(*pd))
+		}
+		if ((ret = pread(fd, pd, sizeof(*pd), offset)) != sizeof(*pd)) {
+			if (ret == -1 && CRASHDEBUG(8))
+				fprintf(fp, "read_pd: pread error: %s\n", strerror(errno));
 			return READ_ERROR;
+		}
 	}
 
 	return 0;
@@ -1125,7 +1131,6 @@ cache_page(physaddr_t paddr)
 	off_t seek_offset;
 	page_desc_t pd;
 	const int block_size = dd->block_size;
-	const off_t failed = (off_t)-1;
 	ulong retlen;
 #ifdef ZSTD
 	static ZSTD_DCtx *dctx = NULL;
@@ -1190,10 +1195,18 @@ cache_page(physaddr_t paddr)
 			return PAGE_INCOMPLETE;
 		}
 	} else {
-		if (lseek(dd->dfd, pd.offset, SEEK_SET) == failed)
+		if (pd.offset < 0) {
+			if (CRASHDEBUG(8))
+				fprintf(fp, "read_diskdump/cache_page: invalid offset: %lx\n",
+					pd.offset);
 			return SEEK_ERROR;
-		if (read(dd->dfd, dd->compressed_page, pd.size) != pd.size)
+		}
+		if ((ret = pread(dd->dfd, dd->compressed_page, pd.size, pd.offset)) != pd.size) {
+			if (ret == -1 && CRASHDEBUG(8))
+				fprintf(fp, "read_diskdump/cache_page: pread error: %s\n",
+					strerror(errno));
 			return READ_ERROR;
+		}
 	}
 
 	if (pd.flags & DUMP_DH_COMPRESSED_ZLIB) {

netdump.c

@@ -4336,7 +4336,7 @@ get_arm_regs_from_elf_notes(struct task_context *tc)
 int
 read_proc_kcore(int fd, void *bufptr, int cnt, ulong addr, physaddr_t paddr) 
 {
-	int i; 
+	int i, ret;
 	size_t readcnt;
 	ulong kvaddr;
 	Elf32_Phdr *lp32;
@@ -4436,11 +4436,16 @@ read_proc_kcore(int fd, void *bufptr, int cnt, ulong addr, physaddr_t paddr)
 	if (offset == UNINITIALIZED)
 		return SEEK_ERROR;
 
-        if (lseek(fd, offset, SEEK_SET) != offset)
-		perror("lseek");
-
-	if (read(fd, bufptr, readcnt) != readcnt)
+	if (offset < 0) {
+		if (CRASHDEBUG(8))
+			fprintf(fp, "read_proc_kcore: invalid offset: %lx\n", offset);
+		return SEEK_ERROR;
+	}
+	if ((ret = pread(fd, bufptr, readcnt, offset)) != readcnt) {
+		if (ret == -1 && CRASHDEBUG(8))
+			fprintf(fp, "read_proc_kcore: pread error: %s\n", strerror(errno));
 		return READ_ERROR;
+	}
 
 	return cnt;
 }