read

glibc's read is in glibc/sysdeps/mach/hurd/read.c:__libc_read.
That calls glibc/hurd/fd-read.c:_hurd_fd_read().
That calls __io_read, which is an RPC, i.e., that actually results into the ext2fs server calling hurd/libdiskfs/io-read.c:diskfs_S_io_read.
That calls _diskfs_rdwr_internal, which calls hurd/libpager/pager-memcpy.c:pager_memcpy, which usually basically just tell the kernel to virtually project the memory object corresponding to the file in the caller process's memory. No read is actually done.
Then, when the process actually reads the data, the kernel gets the user page fault (gnumach/i386/i386/trap.c:user_trap), which calls vm_fault, etc., until actually getting to gnumach/vm/vm_fault/vm_fault_page which eventually calls memory_object_data_request, which is an RPC, i.e., that actually results into the ext2fs server calling hurd/libpager/data-request.c:_pager_seqnos_memory_object_data_request.
That calls hurd/ext2fs/pager.c:pager_read_page, which looks for where the data is on the disk, and eventually calls hurd/libstore/rdwr.c:store_read, which eventually calls device_read, which is an RPC, i.e., that actually gets into the kernel calling gnumach/linux/dev/glue/block.c:device_read.