Add maximum memory limit.

PiperOrigin-RevId: 310179277

4 files changed, 36 insertions, 15 deletions

diff --git a/pkg/sentry/fs/proc/meminfo.go b/pkg/sentry/fs/proc/meminfo.go
index 465b47da9..91617267d 100644
--- a/pkg/sentry/fs/proc/meminfo.go
+++ b/pkg/sentry/fs/proc/meminfo.go
@@ -58,12 +58,16 @@ func (d *meminfoData) ReadSeqFileData(ctx context.Context, h seqfile.SeqHandle)
 
 	var buf bytes.Buffer
 	fmt.Fprintf(&buf, "MemTotal:       %8d kB\n", totalSize/1024)
-	memFree := (totalSize - totalUsage) / 1024
+	memFree := totalSize - totalUsage
+	if memFree > totalSize {
+		// Underflow.
+		memFree = 0
+	}
 	// We use MemFree as MemAvailable because we don't swap.
 	// TODO(rahat): When reclaim is implemented the value of MemAvailable
 	// should change.
-	fmt.Fprintf(&buf, "MemFree:        %8d kB\n", memFree)
-	fmt.Fprintf(&buf, "MemAvailable:   %8d kB\n", memFree)
+	fmt.Fprintf(&buf, "MemFree:        %8d kB\n", memFree/1024)
+	fmt.Fprintf(&buf, "MemAvailable:   %8d kB\n", memFree/1024)
 	fmt.Fprintf(&buf, "Buffers:               0 kB\n") // memory usage by block devices
 	fmt.Fprintf(&buf, "Cached:         %8d kB\n", (file+snapshot.Tmpfs)/1024)
 	// Emulate a system with no swap, which disables inactivation of anon pages.
diff --git a/pkg/sentry/fsimpl/proc/tasks_files.go b/pkg/sentry/fsimpl/proc/tasks_files.go
index 92007df81..e5f13b69e 100644
--- a/pkg/sentry/fsimpl/proc/tasks_files.go
+++ b/pkg/sentry/fsimpl/proc/tasks_files.go
@@ -272,12 +272,16 @@ func (*meminfoData) Generate(ctx context.Context, buf *bytes.Buffer) error {
 	inactiveFile := file - activeFile
 
 	fmt.Fprintf(buf, "MemTotal:       %8d kB\n", totalSize/1024)
-	memFree := (totalSize - totalUsage) / 1024
+	memFree := totalSize - totalUsage
+	if memFree > totalSize {
+		// Underflow.
+		memFree = 0
+	}
 	// We use MemFree as MemAvailable because we don't swap.
 	// TODO(rahat): When reclaim is implemented the value of MemAvailable
 	// should change.
-	fmt.Fprintf(buf, "MemFree:        %8d kB\n", memFree)
-	fmt.Fprintf(buf, "MemAvailable:   %8d kB\n", memFree)
+	fmt.Fprintf(buf, "MemFree:        %8d kB\n", memFree/1024)
+	fmt.Fprintf(buf, "MemAvailable:   %8d kB\n", memFree/1024)
 	fmt.Fprintf(buf, "Buffers:               0 kB\n") // memory usage by block devices
 	fmt.Fprintf(buf, "Cached:         %8d kB\n", (file+snapshot.Tmpfs)/1024)
 	// Emulate a system with no swap, which disables inactivation of anon pages.
diff --git a/pkg/sentry/syscalls/linux/sys_sysinfo.go b/pkg/sentry/syscalls/linux/sys_sysinfo.go
index a65b560c8..297de052a 100644
--- a/pkg/sentry/syscalls/linux/sys_sysinfo.go
+++ b/pkg/sentry/syscalls/linux/sys_sysinfo.go
@@ -29,13 +29,18 @@ func Sysinfo(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysca
 	mf.UpdateUsage()
 	_, totalUsage := usage.MemoryAccounting.Copy()
 	totalSize := usage.TotalMemory(mf.TotalSize(), totalUsage)
+	memFree := totalSize - totalUsage
+	if memFree > totalSize {
+		// Underflow.
+		memFree = 0
+	}
 
 	// Only a subset of the fields in sysinfo_t make sense to return.
 	si := linux.Sysinfo{
 		Procs:    uint16(len(t.PIDNamespace().Tasks())),
 		Uptime:   t.Kernel().MonotonicClock().Now().Seconds(),
 		TotalRAM: totalSize,
-		FreeRAM:  totalSize - totalUsage,
+		FreeRAM:  memFree,
 		Unit:     1,
 	}
 	_, err := t.CopyOut(addr, si)
diff --git a/pkg/sentry/usage/memory.go b/pkg/sentry/usage/memory.go
index 4320ad17f..ab1d140d2 100644
--- a/pkg/sentry/usage/memory.go
+++ b/pkg/sentry/usage/memory.go
@@ -252,18 +252,23 @@ func (m *MemoryLocked) Copy() (MemoryStats, uint64) {
 	return ms, m.totalLocked()
 }
 
-// MinimumTotalMemoryBytes is the minimum reported total system memory.
-//
-// This can be configured through options provided to the Sentry at start.
-// This number is purely synthetic. This is only set before the application
-// starts executing, and must not be modified.
-var MinimumTotalMemoryBytes uint64 = 2 << 30 // 2 GB
+// These options control how much total memory the is reported to the application.
+// They may only be set before the application starts executing, and must not
+// be modified.
+var (
+	// MinimumTotalMemoryBytes is the minimum reported total system memory.
+	MinimumTotalMemoryBytes uint64 = 2 << 30 // 2 GB
+
+	// MaximumTotalMemoryBytes is the maximum reported total system memory.
+	// The 0 value indicates no maximum.
+	MaximumTotalMemoryBytes uint64
+)
 
 // TotalMemory returns the "total usable memory" available.
 //
 // This number doesn't really have a true value so it's based on the following
-// inputs and further bounded to be above some minimum guaranteed value (2GB),
-// additionally ensuring that total memory reported is always less than used.
+// inputs and further bounded to be above the MinumumTotalMemoryBytes and below
+// MaximumTotalMemoryBytes.
 //
 // memSize should be the platform.Memory size reported by platform.Memory.TotalSize()
 // used is the total memory reported by MemoryLocked.Total()
@@ -279,5 +284,8 @@ func TotalMemory(memSize, used uint64) uint64 {
 			memSize = uint64(1) << (uint(msb) + 1)
 		}
 	}
+	if MaximumTotalMemoryBytes > 0 && memSize > MaximumTotalMemoryBytes {
+		memSize = MaximumTotalMemoryBytes
+	}
 	return memSize
 }